Fix GcObject to_json mutable borrow panic

[0x7D2B]

This Pull Request fixes #1059. The object was getting borrowed in to_json to iterate over its keys, preventing mutable borrows. The fix was to collect the keys first and to iterate over them. This will also make the test behave as expected as it will prevent properties added during iteration from being enumerated.

[github-actions]

Test262 conformance changes:

Test result	master count	PR count	difference
Total	78,873	78,873	0
Passed	26,502	26,502	0
Ignored	15,604	15,604	0
Failed	36,767	36,767	0
Panics	27	24	-3
Conformance	33.60%	33.60%	0.00%

[github-actions]

Benchmark for aad1c8e

Click to view benchmark

Test	PR Benchmark	Master Benchmark	%
Arithmetic operations (Execution)	334.3±13.72ns	324.1±14.96ns	+3.15%
Arithmetic operations (Full)	249.5±9.22µs	241.8±10.37µs	+3.18%
Array access (Execution)	5.9±0.22µs	5.5±0.28µs	+7.27%
Array access (Full)	259.0±12.56µs	260.9±12.68µs	-0.73%
Array creation (Execution)	2.7±0.10ms	2.7±0.07ms	0.00%
Array creation (Full)	2.9±0.12ms	2.9±0.12ms	0.00%
Array pop (Execution)	869.4±26.90µs	838.9±26.77µs	+3.64%
Array pop (Full)	1288.9±49.07µs	1305.8±81.59µs	-1.29%
Boolean Object Access (Execution)	4.9±0.18µs	4.7±0.23µs	+4.26%
Boolean Object Access (Full)	267.8±9.61µs	261.6±11.71µs	+2.37%
Clean js (Execution)	610.8±25.49µs	603.3±25.37µs	+1.24%
Clean js (Full)	896.8±32.29µs	881.2±35.76µs	+1.77%
Clean js (Parser)	36.0±1.96µs	37.3±1.65µs	-3.49%
Create Realm	408.8±8.84ns	392.0±20.06ns	+4.29%
Dynamic Object Property Access (Execution)	4.7±0.22µs	4.4±0.18µs	+6.82%
Dynamic Object Property Access (Full)	256.3±12.23µs	272.2±12.36µs	-5.84%
Expression (Parser)	6.7±0.30µs	6.7±0.36µs	0.00%
Fibonacci (Execution)	704.3±20.67µs	686.6±28.03µs	+2.58%
Fibonacci (Full)	984.3±33.34µs	948.9±36.77µs	+3.73%
For loop (Execution)	20.7±0.69µs	19.4±0.90µs	+6.70%
For loop (Full)	272.0±13.48µs	275.4±14.01µs	-1.23%
For loop (Parser)	18.5±0.78µs	18.2±0.66µs	+1.65%
Goal Symbols (Parser)	12.8±0.50µs	12.7±0.54µs	+0.79%
Hello World (Parser)	3.6±0.14µs	3.7±0.18µs	-2.70%
Long file (Parser)	711.0±32.13ns	697.4±29.60ns	+1.95%
Mini js (Execution)	542.5±21.38µs	541.3±26.86µs	+0.22%
Mini js (Full)	853.7±29.87µs	854.4±25.84µs	-0.08%
Mini js (Parser)	33.7±1.23µs	32.9±1.48µs	+2.43%
Number Object Access (Execution)	3.9±0.13µs	3.8±0.14µs	+2.63%
Number Object Access (Full)	255.9±10.33µs	257.5±11.40µs	-0.62%
Object Creation (Execution)	4.1±0.15µs	3.9±0.18µs	+5.13%
Object Creation (Full)	255.3±11.18µs	256.9±10.81µs	-0.62%
RegExp (Execution)	11.2±0.51µs	10.7±0.51µs	+4.67%
RegExp (Full)	269.1±9.57µs	271.0±11.83µs	-0.70%
RegExp Literal (Execution)	10.9±0.61µs	11.2±0.33µs	-2.68%
RegExp Literal (Full)	263.3±11.83µs	269.4±12.04µs	-2.26%
RegExp Literal Creation (Execution)	9.8±0.41µs	9.5±0.40µs	+3.16%
RegExp Literal Creation (Full)	272.7±10.31µs	264.1±10.71µs	+3.26%
Static Object Property Access (Execution)	4.3±0.17µs	4.1±0.18µs	+4.88%
Static Object Property Access (Full)	270.4±6.57µs	260.7±13.02µs	+3.72%
String Object Access (Execution)	6.8±0.33µs	6.5±0.33µs	+4.62%
String Object Access (Full)	272.1±9.67µs	261.4±10.71µs	+4.09%
String comparison (Execution)	5.8±0.26µs	6.0±0.28µs	-3.33%
String comparison (Full)	261.6±14.40µs	253.6±15.60µs	+3.15%
String concatenation (Execution)	4.7±0.21µs	4.7±0.21µs	0.00%
String concatenation (Full)	251.3±11.16µs	259.6±11.71µs	-3.20%
String copy (Execution)	3.6±0.15µs	3.6±0.13µs	0.00%
String copy (Full)	249.3±11.89µs	240.7±11.12µs	+3.57%
Symbols (Execution)	3.2±0.11µs	3.0±0.16µs	+6.67%
Symbols (Full)	241.7±10.83µs	245.4±13.11µs	-1.51%

[Razican]

This is good from my side. Hopefully performance won't suffer a lot.