Fix MIRI under tree borrows

[djkoloski]

This change consists of four modifications to fix the test suite when run under miri with MIRIFLAGS="-Zmiri-disable-stacked-borrows -Zmiri-tree-borrows":

1. BitSlice now contains a trailing slice of [UnsafeCell<()>] rather than [()]. This corrects the pointer aliasing semantics for BitSlice so that the self reference does not assert shared or mutable aliasing.
2. Locations which casted usizes back to pointers have been switched to use the unstable ptr::map_addr function when testing through miri. This function preserves the provenance of the pointer across bit manipulations. This fixes some warnings.
3. A manual impl for RefUnwindSafe is now provided for BitSlice. I didn't think too hard about this but I'm pretty sure it's right.
4. Some BitPtr tests have been fixed up to re-acquire pointers after mutating operations. This re-asserts mutable access to the underlying memory with the correct semantics for tree borrows.

[JoJoDeveloping]

note that -Zmiri-disable-stacked-borrows is unnecessary when combined with -Zmiri-tree-borrows.