It would be nice if we could directly instantiate OffsetMaps. While we currently provide copyOf() operations, it would be nice if the users could, use a scratch space builder, where they pre-assign offsets.

This is useful for situations where the keySet is known and we are performing a transformation - hence a transformedFrom() would be sufficient, but perhaps we can provide an efficient way of caching the key->offset map, too, so that that lookup/cache is not needed.