Boxed Bit-Slice Iteration

This module contains the by-value iterator used by both BitBox and BitVec. In the standard library, this iterator is defined under alloc::vec, not alloc::boxed, as Box already has an iteration implementation that forwards to its boxed value.

It is moved here for simplicity: both BitBox and BitVec iterate over a dynamic bit-slice by value, and must deällocate the region when dropped. As BitBox has a smaller value than BitVec, it is used as the owning handle for the bit-slice being iterated.