Merge Const/Let/Var DeclList into DeclarationList

[0x7D2B]

Currently Boa implements declaration lists as three separate structs (LetDeclList, ConstDeclList, VarDeclList) containing three separate structs for individual declarations (LetDecl, ConstDecl, VarDecl).

With a lot of code code duplication between the three declaration list implementations and borderline identical interfaces provided, it seems like a good idea to combine them.

For reference, the original spec treats Const and Let as a single type of declaration: https://tc39.es/ecma262/#sec-declarations-and-the-variable-statement - this commit goes one step further and also combines them with Var.

The new declaration is implemented as an enum with Const, Let, and Var as its variants. Each of these variants wraps a Box<[Declaration]>, similar to how the original separate structs worked. I opted to go for this implementation in anticipation of enum variant types.

Merging these structs will also simplify iteration over declaration lists, since they all now use the same objects. This will be useful for implementing features such as #1175 which make use of bound name lists.

[0x7D2B]

no test regressions:

Test262 conformance changes:

Test result	master count	PR count	difference
Total	78,497	78,497	0
Passed	25,344	25,344	0
Ignored	15,587	15,587	0
Failed	37,566	37,566	0
Panics	16	16	0
Conformance	32.29	32.29	0.00%

[jasonwilliams]

This looks good to me I have no issues, thanks for working on this @0x7D2B

[jasonwilliams]

┌─────────┬──────────────────────────────────────────────┬──────────────────────┬─────────────────────┬────────────┐
│ (index) │                     name                     │   changesDuration    │   masterDuration    │ difference │
├─────────┼──────────────────────────────────────────────┼──────────────────────┼─────────────────────┼────────────┤
│    0    │     'Arithmetic operations (Execution)'      │   '370.6±28.00ns'    │ '**323.1±35.23ns**' │   '+15'    │
│    1    │        'Arithmetic operations (Full)'        │   '278.9±22.75µs'    │ '**264.1±32.91µs**' │   '+6.0'   │
│    2    │          'Array access (Execution)'          │   '**5.9±0.42µs**'   │    '6.3±0.68µs'     │   '-6.5'   │
│    3    │            'Array access (Full)'             │   '301.0±47.62µs'    │ '**262.0±19.41µs**' │   '+15'    │
│    4    │         'Array creation (Execution)'         │     '2.4±0.14ms'     │  '**2.4±0.15ms**'   │   '+1.0'   │
│    5    │           'Array creation (Full)'            │     '2.8±0.20ms'     │  '**2.7±0.17ms**'   │   '+2.0'   │
│    6    │           'Array pop (Execution)'            │   '924.6±92.35µs'    │ '**776.3±51.67µs**' │   '+19'    │
│    7    │              'Array pop (Full)'              │ '**1183.1±73.54µs**' │  '1237.7±93.11µs'   │   '-4.8'   │
│    8    │     'Boolean Object Access (Execution)'      │     '4.9±0.30µs'     │  '**4.8±0.33µs**'   │   '+1.0'   │
│    9    │        'Boolean Object Access (Full)'        │ '**249.1±16.81µs**'  │   '255.2±16.89µs'   │   '-2.0'   │
│   10    │            'Clean js (Execution)'            │   '729.7±28.17µs'    │ '**625.2±56.10µs**' │   '+17'    │
│   11    │              'Clean js (Full)'               │   '1035.3±53.93µs'   │ '**918.7±72.50µs**' │   '+13'    │
│   12    │             'Clean js (Parser)'              │    '43.5±4.52µs'     │  '**38.5±2.95µs**'  │   '+13'    │
│   13    │                'Create Realm'                │   '416.0±27.46ns'    │ '**410.9±38.77ns**' │   '+1.0'   │
│   14    │ 'Dynamic Object Property Access (Execution)' │     '5.5±0.50µs'     │  '**4.9±0.33µs**'   │   '+11'    │
│   15    │   'Dynamic Object Property Access (Full)'    │ '**246.4±13.98µs**'  │   '261.1±22.95µs'   │   '-5.7'   │
│   16    │            'Expression (Parser)'             │   '**6.2±0.40µs**'   │    '7.0±0.42µs'     │   '-12'    │
│   17    │           'Fibonacci (Execution)'            │   '869.4±46.44µs'    │   '870.0±73.76µs'   │   '0.0'    │
│   18    │              'Fibonacci (Full)'              │ '**1104.0±71.28µs**' │  '1151.5±89.51µs'   │   '-3.8'   │
│   19    │            'For loop (Execution)'            │    '23.4±2.29µs'     │  '**22.5±2.50µs**'  │   '+4.0'   │
│   20    │              'For loop (Full)'               │ '**287.3±21.17µs**'  │   '324.4±29.10µs'   │   '-12'    │
│   21    │             'For loop (Parser)'              │  '**19.0±2.05µs**'   │    '19.6±2.08µs'    │   '-2.9'   │
│   22    │           'Goal Symbols (Parser)'            │    '13.4±0.97µs'     │  '**13.1±0.93µs**'  │   '+2.0'   │
│   23    │            'Hello World (Parser)'            │     '3.9±0.44µs'     │    '3.9±0.20µs'     │   '0.0'    │
│   24    │             'Long file (Parser)'             │   '681.5±48.15ns'    │ '**676.2±44.27ns**' │   '+1.0'   │
│   25    │            'Mini js (Execution)'             │ '**556.8±40.04µs**'  │   '619.9±43.42µs'   │   '-9.9'   │
│   26    │               'Mini js (Full)'               │   '995.3±57.85µs'    │ '**812.9±52.37µs**' │   '+22'    │
│   27    │              'Mini js (Parser)'              │    '39.7±5.63µs'     │  '**33.3±2.02µs**'  │   '+19'    │
│   28    │      'Number Object Access (Execution)'      │     '3.9±0.25µs'     │  '**3.8±0.26µs**'   │   '+1.0'   │
│   29    │        'Number Object Access (Full)'         │   '252.7±22.89µs'    │ '**251.4±17.89µs**' │   '+1.0'   │
│   30    │        'Object Creation (Execution)'         │     '4.3±0.37µs'     │  '**4.1±0.24µs**'   │   '+4.0'   │
│   31    │           'Object Creation (Full)'           │   '265.3±53.31µs'    │ '**255.0±16.65µs**' │   '+4.0'   │
│   32    │             'RegExp (Execution)'             │  '**10.3±0.84µs**'   │    '10.7±1.42µs'    │   '-3.8'   │
│   33    │               'RegExp (Full)'                │ '**265.9±19.41µs**'  │   '282.2±25.27µs'   │   '-5.7'   │
│   34    │         'RegExp Literal (Execution)'         │  '**10.2±0.79µs**'   │    '10.3±0.73µs'    │  '-0.99'   │
│   35    │           'RegExp Literal (Full)'            │ '**260.0±16.74µs**'  │   '267.4±21.68µs'   │   '-2.9'   │
│   36    │    'RegExp Literal Creation (Execution)'     │     '9.0±0.77µs'     │    '9.0±0.70µs'     │   '0.0'    │
│   37    │       'RegExp Literal Creation (Full)'       │   '263.0±19.68µs'    │ '**254.1±18.15µs**' │   '+3.0'   │
│   38    │ 'Static Object Property Access (Execution)'  │   '**4.5±0.36µs**'   │    '4.5±0.30µs'     │  '-0.99'   │
│   39    │    'Static Object Property Access (Full)'    │ '**250.2±17.69µs**'  │   '252.4±16.29µs'   │  '-0.99'   │
│   40    │      'String Object Access (Execution)'      │     '7.0±0.41µs'     │  '**6.7±0.43µs**'   │   '+4.0'   │
│   41    │        'String Object Access (Full)'         │ '**257.9±17.19µs**'  │   '266.2±24.17µs'   │   '-2.9'   │
│   42    │       'String comparison (Execution)'        │     '6.7±0.49µs'     │  '**6.4±0.54µs**'   │   '+5.0'   │
│   43    │          'String comparison (Full)'          │   '264.6±21.58µs'    │ '**261.6±32.69µs**' │   '+1.0'   │
│   44    │      'String concatenation (Execution)'      │     '5.1±0.47µs'     │  '**4.9±0.40µs**'   │   '+4.0'   │
│   45    │        'String concatenation (Full)'         │   '262.2±17.94µs'    │ '**254.4±21.96µs**' │   '+3.0'   │
│   46    │          'String copy (Execution)'           │     '4.2±0.45µs'     │  '**3.8±0.29µs**'   │   '+11'    │
│   47    │             'String copy (Full)'             │ '**252.9±25.57µs**'  │   '257.7±24.94µs'   │   '-2.0'   │
│   48    │            'Symbols (Execution)'             │     '3.5±0.35µs'     │  '**3.4±0.27µs**'   │   '+4.0'   │
│   49    │               'Symbols (Full)'               │ '**262.3±22.01µs**'  │   '274.9±12.67µs'   │   '-4.8'   │
│   50    │                      ''                      │      undefined       │      undefined      │   '+NaN'   │
└─────────┴──────────────────────────────────────────────┴──────────────────────┴─────────────────────┴────────────┘