Auto merge of #55270 - RalfJung:stacked-borrows-ng, r=oli-obk

Auto merge of #55270 - RalfJung:stacked-borrows-ng, r=oli-obk

miri engine: Stacked Borrows NG

For more refined tracking in miri, we do return untagged pointers from the memory abstraction after allocations and let the caller decide how to tag these.

Also refactor the `tag_(de)reference` hooks so they can be more easily called in the ref-to-place and place-to-ref methods, and reorder things in validation: validation calls ref-to-place which (when running in miri) triggers some checks, so we want to run it rather late and catch other problems first. We also do not need to redundantly check the ref to be allocated any more, the checks miri does anyway imply thath.

r? @oli-obk