A two-way map data structure for cloneable keys and values.
This is currently pretty minimal; it doesn't yet support iterators, for instance.
Most functions come in _fwd and _rev variants; where the _fwd varaint acts on the second
entry given the first, and _rev is the opposite.
This crate is best for values that are cheap to clone, since internally it stores two copies of each element. To use it with large values, consider wrapping them in `Rc to make them cheap to clone.