Rollup merge of #122537 - RalfJung:interpret-allocation, r=oli-obk

interpret/allocation: fix aliasing issue in interpreter and refactor getters a bit

That new raw getter will be needed to let Miri pass pointers to natively executed FFI code ("extern-so" mode).

While doing that I realized our get_bytes_mut are named less scary than get_bytes_unchecked so I rectified that. Also I realized `mem_copy_repeatedly` would break if we called it for multiple overlapping copies so I made sure this does not happen.

And I realized that we are actually [violating Stacked Borrows in the interpreter](https://rust-lang.zulipchat.com/#narrow/stream/136281-t-opsem/topic/I.20think.20Miri.20violates.20Stacked.20Borrows.20.F0.9F.99.88).^^ That was introduced in rust-lang/rust#87777.

r? ```@oli-obk```

Author: matthiaskrgr