allocator_api

This is a copy of the unstable allocator_api (https://github.com/rust-lang/rust/issues/32838) and of parts of the unstable alloc feature.

Usable with stable rust, but requires 1.25.

Differences with nightly rust

The code was copied from the rust repository as of 8dd24c8ed4ac3e48068408fa21d491d7ffe45295, with #[stable] and #[unstable] annotations removed.

In the allocator module (corresponding to core::heap), the oom function infinitely loops instead of calling core::intrinsics::abort, which is not stable. Implementations of the trait should override oom to handle the situation more appropriately.

In the rawvec module (corresponding to alloc::rawvec), RawVec uses NonNull instead of Unique.

In the boxed module (corresponding to alloc::boxed), the Box type is augmented such that it is associated with an allocator, similarly to RawVec. Its API is consequently slightly different from std::boxed::Box (e.g. from_raw is replaced with from_raw_in). The same (stable) features as std::boxed::Box are implemented, except for downcast for Box<Any> and Box<Any + Send>, and Box<str> functions. Like for RawVec, the type relies on NonNull rather than Unique.

Caveat: the types provided in this crate cannot be used where the corresponding types from std/alloc are expected. Few APIs should be taking those types directly as input, though.