Remove ~const from alloc
There is currently an effort underway to stop using `~const Trait`, temporarily, so as to refactor the logic underlying const traits with relative ease. This means it has to go from the standard library, as well.
I have taken the initial step of just removing these impls from alloc, as removing them from core is a much more tangled task. In addition, all of these implementations are one more-or-less logically-connected group, so reverting their deconstification as a group seems like it will also be sensible.
r? `@fee1-dead`
c62bb46fb3