alloc sans `alloc`

[ds84182]

Given that the Allocator trait is in core, shouldn't the various allocatable collections be available in core as well?

e.g. Vec<T, A> and Box<T, A> in core without the fns that require A = GlobalAlloc (so no Vec::new, only Vec::new_in).

This would solve #123.

[CAD97]

This unfortunately effectively requires solving at least inference-affecting defaults in practice, which is a very difficult problem.

We could potentially break coherence within the std facade to accomplish this, but we've generally been hesitant to do so beyond what already exists, at least without first developing a way for non-std crates to do so.