Add fn replace_aliased(&self, usize, bool) -> bool to BitSlice<T, O>

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This PR adds the following methods to BitSlice<T, O>:

• fn replace_aliased(&self, usize, bool) -> bool
• fn replace_aliased_unchecked(&self, usize, bool) -> bool

They are modeled after the existing fn set_aliased()/fn set_aliased_unchecked() methods on BitSlice<T, O>.

The motivation behind these methods is to be able to use BitVec<AtomicUsize> for keeping track of "visited" nodes in a parallel graph traversal without the need of locking. As such you often need to atomically add a node to the "visited" set, while also find out if it had already been visited before (e.g. for avoiding performing redundant computations).

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Just noticed this other PR that sort of aims to achieve the same: #273