feat(allocator,ast): add BoxWithDrop<BigInt>

[Boshen]

relates #1803

I wanna make things work first, get miri to pass, and then work on better workarounds. cc @overlookmotel

Miri run: https://github.com/oxc-project/oxc/actions/runs/7693186484

[Boshen]

Ohhh... this won't work, the drop calls need to be chained all the way to the top :-) I can't just stick a BoxWithDrop in the middle of no where and expect it's drop to be called.

[codspeed-hq]

CodSpeed Performance Report

Merging #2201 will not alter performance

_{⚠️ No base runs were found}

_{Falling back to comparing bigint-box-with-drop (fb3d877) with main (56adfb1)}

Summary

✅ 17 untouched benchmarks

[overlookmotel]

The best solution I think is to prevent BigInt from making allocations in the first place. Unfortunately that'd require reimplementing BigInt (or at least the parts of its API which OXC uses).

To do a swifter fix in the meantime, the only way I can see to fix the memory leak without making the entire AST drop is:

1. Store pointers to all BigInts in a vec in Allocator.
2. Add impl Drop to Allocator which converts those pointers back to BigInts and drops them.

That would solve the leak, though I'm not sure it'd be sound - the pointers would need to be *mut, so it might well be violating Rust's aliasing rules.