Give finite maps their size function without an adjoin_to_theory call

This call had broken because the parsing of the term within the
...Theory.sml broke with 0801c6f226. Luckily such calls are not
necessary in the presence of TypeBase.export.

With a regression test.

Fix more regression tests broken by testutils API change

Import alist_tree theory and library from CakeML

Move src/enumfset material into src/finite_maps

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.