Implement erased objects

[jedel1043]

This uses the changes made in #3491 and takes them to the extreme. For a quick rundown, this changes our
current definition of Object from:

pub struct Object {
    kind: ObjectKind,
    properties: PropertyMap,
    extensible: bool,
    private_elements: ThinVec<(PrivateName, PrivateElement)>,
}

to:

pub struct Object<T: ?Sized> {
    pub(crate) properties: PropertyMap,
    pub(crate) extensible: bool,
    private_elements: ThinVec<(PrivateName, PrivateElement)>,
    data: T,
}

Which allows casting from a specific Object<T> to an erased Object<dyn NativeObject>, enabling storing them inside a JsObject.

It comes with some very big memory wins overall! Most of these results were generated using small modifications of the Quick JS benchmark.

Heap usage (Main)

dhat: Total: 5,297,772,730 bytes in 42,698,955 blocks
dhat: At t-gmax: 10,021,109 bytes in 68,943 blocks
dhat: At t-end: 8,229,723 bytes in 62,670 blocks

Heap usage (PR)

dhat: Total: 4,344,956,646 bytes in 42,734,151 blocks
dhat: At t-gmax: 9,899,221 bytes in 68,946 blocks
dhat: At t-end: 7,138,579 bytes in 62,072 blocks

Not everything is good news though. I benchmarked the performance after this change and it degraded considerably. I was really confused by this regression, so I profiled a smaller version of the Splay benchmark to see the points where the program was spending the most time, but...

Main

https://share.firefox.dev/414oQ66

PR

https://share.firefox.dev/3t2vQno

The size wins are so big that the GC is increasing its threshold more slowly, which increases the number of collections overall!
This is extra sad because Collector::collect takes 46ms to run on main, while this PR reduces its runtime to 37 ms 😭.

If we want to recover the lost performance, we'll need to design more complex heuristics that trigger garbage collection less often, but I would very much prefer to make that change in another PR, because this PR is already pretty massive.

NOTE: Implement erased objects is the commit containing the foundation of the PR; the commits succeeding it are mostly patches to migrate our code to the new design.

NOTE 2: I'm marking this as draft because it's still missing some changes like documentation or fixing our examples, but I wanted to start the reviews as soon as possible. Done.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	95,609	95,609	0
Passed	76,526	76,526	0
Ignored	18,132	18,132	0
Failed	951	951	0
Panics	0	0	0
Conformance	80.04%	80.04%	0.00%

[HalidOdat]

Really nice work! Really like the new design! :)

Just some minor nitpicks and questions.

As for the the threshold thrashing we may want to decrease the used_space_percentage from 80 to something like 70 (or something simular) rust-gc has is it set to 70 percent.

Ran the benchmarks with that ratio and saw that the speed is more or less the same with main. We can set it to that ratio while we figure out a better way to calculate better thresholds.

[codecov]

Codecov Report

Attention: 1454 lines in your changes are missing coverage. Please review.

Comparison is base (c113b74) 46.42% compared to head (eca8048) 46.83%.

Files	Patch %	Lines
boa_engine/src/builtins/proxy/mod.rs	5.05%	338 Missing ⚠️
...src/builtins/typed_array/integer_indexed_object.rs	0.00%	137 Missing ⚠️
boa_engine/src/builtins/temporal/calendar/mod.rs	21.05%	135 Missing ⚠️
boa_engine/src/builtins/typed_array/builtin.rs	0.00%	97 Missing ⚠️
boa_engine/src/object/mod.rs	27.17%	67 Missing ⚠️
boa_engine/src/builtins/function/arguments.rs	28.20%	56 Missing ⚠️
boa_engine/src/builtins/function/bound.rs	0.00%	54 Missing ⚠️
boa_engine/src/builtins/temporal/instant/mod.rs	0.00%	45 Missing ⚠️
boa_engine/src/value/display.rs	57.83%	35 Missing ⚠️
boa_macros/src/lib.rs	0.00%	28 Missing ⚠️
... and 69 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3494      +/-   ##
==========================================
+ Coverage   46.42%   46.83%   +0.41%     
==========================================
  Files         496      493       -3     
  Lines       50775    50077     -698     
==========================================
- Hits        23570    23455     -115     
+ Misses      27205    26622     -583     

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[jedel1043]

Not really worried about coverage since this doesn't add any new builtins. Maybe running the examples would increase our coverage though.

[HalidOdat]

Looks good to me! Just some small nitpicks :)

[raskad]

Very nice! I also like moving the internal methods to their builtins.