Return `Bump` from collections

[zetanumbers]

I would argue that it is our best to name methods like in allocator_api feature (Box::allocator, Box::into_inner_with_allocator, etc.)

[fitzgen]

Not sure it is worth copying more of std here, rather than just waiting for the allocator API to stabilize.

[zetanumbers]

This is mostly for ergonomics, for example to eliminate creation of arbitrary lifetimes for from_raw* functions.