Improve on indexed maps primary key / index keys helpers

[maurolacy]

It would be nice to have a key(K) method handy for every indexed map type (IndexedMap and IndexedSnapshotMap, currently), so that building a range() (primary) key is easier, and well documented.

The same for every index, i.e. a key(IK) (or index_key(IK), for clarity) for every index an indexed map has, so that building a range() key over an index is easy.

key(K) is already implemented for Map, and probably other map types. Creating this issue to extend / consolidate functionality accross all map types and their indexes.