Move `core::alloc::MemoryBlock` to `core::mem::Block`?

[TimDiekmann]

Similar to #59, but less complicated, as MemoryBlock is gated behind allocator_api.

[Amanieu]

I feel that this should stay in alloc since it is pretty tied to memory allocation.

[TimDiekmann]

It's currently used only for allocation, yes, but this also applies to #59. This is why I'm asking 🙂

[TimDiekmann]

Closed for the same reason as in rust-lang/rust#71856