Consider using the allocator_api crate

[fitzgen]

Instead of forking from libstd directly ourselves.

This would involve merging String upstream into that crate plus any other collections we might end up needing.

[glandium]

Note that I'm open to having more collections in the allocator_api. I didn't come to it because I didn't need them yet.

[glandium]

Also note that the last published version is behind recent API changes to libstd, but I updated master recently.