Don't enable allocator-api2/nightly feature

[NeuralModder]

See #564 for rationale. Tentatively fixes said issue and #483, though the root cause is up to the allocator-api2 crate to fix.

Direct use of allocator-api2 in tests had to be replaced with the indirection used elsewhere in the crate.

[Amanieu]

I'm not sure what's up with miri but it seems related to the allocator API. I'm away this week so I can't look into it myself.

[NeuralModder]

@Amanieu I fixed the miri (actually a cargo test) thing, the tests were using allocator-api2 directly but there must've been some kind of commingling of the two apis when nightly was enabled, replacing the calls to allocator-api2 with calls to crate::raw fixed it.

What's failing now is tests/set.rs due to a linker warning, but only on aarch64-apple-darwin (or at least, not on linux), possibly only on nightly. My best guess is that this a platform-specific linker oddity, but of course, I have no idea why this PR would cause it. I unfortunately do not have a cross-compilation environment set up to debug this, either.

EDIT: Re-ran CI (pushed revert commit and removed it again) and it passes now, so it was probably something like a random problem with a runner and had nothing to do with the actual contents of this PR.