Allow OffsetMaps to be efficiently instantiated

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.