This PR adds lemmas for the `TreeMap` operations `filter`, `map` and
`filterMap`. These lemmas existed already for hash maps and are simply
ported over from there.
This PR adds the types `Std.ExtDTreeMap`, `Std.ExtTreeMap` and
`Std.ExtTreeSet` of extensional tree maps and sets. These are very
similar in construction to the existing extensional hash maps with one
exception: extensional tree maps and sets provide all functions from
regular tree maps and sets. This is possible because in contrast to hash
maps, tree maps are always ordered.
