HashMap using RawTable from hashbrown

[hansihe]

This is a copy/paste of the HashMap implementation from hashbrown.

This depends on rust-lang/hashbrown#133, which adds allocator support to hashbrown's RawTable. Do not merge before this lands.

This is split into two parts:

1. Add a unstable_core_alloc feature that uses the Alloc trait in std instead of the copied internal version.
2. Add a collections_hash feature that depends on both the unstable_core_alloc feature and the hashbrown crate. I thought it was best to add a new feature for this since it adds a whole new dependency.

Bikeshedding of the feature names is encouraged. If this is a welcome addition, I would be open to adding HashSet as well, which should require significantly less code.

[fitzgen]

Thanks for your work on this.

However, I don't feel comfortable maintaining this code. I think a better path forward would be to push on getting custom allocators in libstd and in the meantime, get this crate and hashbrown both working with the allocator_api crate's trait (a copy of the unstable Alloc trait from liballoc), so that you could connect the two without either this repo or hashbrown needing to maintain/fork a bunch of code.

[TethysSvensson]

I am also doubtful of whether this would actually be useful in practice, since bumpalo is not well-suited for applications in need of frequent re-allocations.

[fitzgen]

Closing this as explained in #54 (review)

[Amanieu]

I'm happy to accept a PR adding allocator_api support to hashbrown. You can extend your existing PR (rust-lang/hashbrown#133) to add allocator support to the full HashMap API instead of just the internal RawTable API.