adding feature to suport nightly features on dependencies

[jakubDoka]

this resolves problems with feature unification
and resolves #209

[jakubDoka]

https://github.com/bytecodealliance/regalloc2/actions/runs/12948971873/job/36118715209?pr=210 @cfallin this workflow most likely needs an update

[cfallin]

@jakubDoka feel free to update the GitHub CI configuration too: the all-features check can probably instead become a list of cargo check invocations with specific feature sets, e.g. no default features; std; std, checker; std, trace-log; std, serde; serde. Or something like that? Then separately a cargo +nightly check --features allocator-api?

[hanna-kruppe]

The need to do this has been resolved by updates to hashbrown (rust-lang/hashbrown#606 solves the specific problem with hashbrown/nightly) and allocator-api2 (zakarumych/allocator-api2#30 solves it for everyone else).

[Amanieu]

This should now be fixed with the latest release of hashbrown.