Move l2n theorems from patricia_casts to numposrep

Drop l2n_LENGTH in the process, covered by l2n_lt.

Get src/finite_maps to build with tight equalities

Start merging finite_map/dictionary theories into one directory

Aim is to also include src/enumfset, possibly also the balanced_bst
development from examples, and a CakeML theory (alist_tree).

With all this stuff in the same place, we reduce branching under src/
and get better concurrency when building.