Make the allocator_api and allocator-api2 features compatible

[AlexanderPortland]

In some niche use cases, it seems useful for &Bump to implement both core::alloc::Allocator and allocator-api2::alloc::Allocator at once. This has come up for me specifically when using data structures from different crates which were generic over those differing allocator traits.

However, simply enabling both the "allocator_api" and "allocator-api2" features doesn't work because there's only a single impl block that's conditionally compiled if either feature is enabled. This causes the "allocator_api" implementation to overwrite the "allocator-api2" one.

If this is intended behavior, would it be alright if I add a note to the README to that effect to help others who might be confused like I was initially? And if not, this PR just introduces two separate impl blocks so that &Bump can satisfy both traits simultaneously.

[fitzgen]

The allocator_api2::alloc::Allocator trait should become just a re-export of the core::alloc::Allocator trait when using nightly and enabling the unstable allocator-api Rust feature. If things aren't working as expected, happy to help clarify and brainstorm fixes.