Fix MIRI under tree borrows #266
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This change consists of four modifications to fix the test suite when run under miri with
MIRIFLAGS="-Zmiri-disable-stacked-borrows -Zmiri-tree-borrows":BitSlicenow contains a trailing slice of[UnsafeCell<()>]rather than[()]. This corrects the pointer aliasing semantics forBitSliceso that the self reference does not assert shared or mutable aliasing.usizes back to pointers have been switched to use the unstableptr::map_addrfunction when testing through miri. This function preserves the provenance of the pointer across bit manipulations. This fixes some warnings.RefUnwindSafeis now provided forBitSlice. I didn't think too hard about this but I'm pretty sure it's right.BitPtrtests 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.