P0009: Add follow-on: inverse layout mapping

A colleague requested that all unique layout mappings have an inverse mapping function. This would map a 1-D offset to the corresponding multidimensional index tuple, if the offset corresponds to a valid multidimensional index tuple.

My colleague proposed it as a nonmember function, constrained on is_always_unique() and preconditioned on is_unique().

An interesting use case would be to change iteration order in a flattened loop. For example, one might be iterating over [0, required_span_size()) on a 2-D square domain, but might want to view the domain using a 2-D space-filling curve. One could do this by inverting the layout_left mapping, and then giving the resulting (i,j) index pair to a space-filling curve's layout mapping.

Start by reviewing the proposed inverse mapping requirements, including valid multidimensional index tuples, required_span_size(), is_always_unique(), and is_unique(). Define the nonmember API and its behavior for offsets that do not correspond to valid indices; done means the proposal resolves the interface and preconditions for changing iteration order.