Support parametric allocators

[joshlf]

Add support for passing a custom allocator with the new allocator API.

[Des-Nerger]

Just to note: such capability doesn't necessarily have to be nightly-only. E.g. hashbrown is a good example of unifying both the nightly-rust allocator-api feature and stable-rust allocator-api2 crate under the same interface. (On the allocator implementation side of things, bumpalo does support allocator-api2, too.)

[alejandro-vaz]

yeah, implementation is on #387