Add support for the Rust's unstable allocator-api on Vec

[aabizri]

This PR adds feature-gated support for rust's new(ish) allocator api on Vec (see rust-lang/rust#32838 & https://doc.rust-lang.org/unstable-book/library-features/allocator-api.html).

This enables support for custom allocators, which are often needed in video games & high performance projects, as well as make it possible for libraries supporting the API to use encase with user-provided Vecs. I haven't added support for Box<T> which now can also take a custom allocator, as that feature still sometime causes ICEs.

As for the feature name, I chose allocator_api, as that's also the name of Rust's feature (and it's disabled by default).

This takes the form of an additional bound on Vecs, which now look like Vec<T, A> where A: std::alloc::Allocator.

[aabizri]

It seems that clippy & MSRV checks are failing as they enable all features, and allocator_api is unstable (and thus not usable on stable). If @teoxoy would like to go-ahead with this new feature, those checks will need to swap --all-features with --features mint,cgmath,glam,nalgebra,ultraviolet,vek,smallvec,arrayvec,tinyvec,ndarray,rpds,archery,im,im-rc,imbl,static-rc.

[teoxoy]

Thanks for the PR!

To avoid the code duplication could you try something like this:

impl<#[cfg(feature = "allocator_api")] A: std::alloc::Allocator>
ByteVecExt for Vec<u8, #[cfg(feature = "allocator_api")] A>

Also, I feel like we should also extend this PR to cover all traits implemented for Vec (ShaderType, ShaderSize, ...)

On a related note, do you know how close this is to stabilization?

[teoxoy]

@aabizri are you still interested in pursuing this?