Move some of the machinery behind BasicProvers.VAR_EQ_TAC to Tactic

Move some term-equality helper functions from boolLib to boolSyntax

This helps with dependency chains and provides some documentation in
that they now appear in a signature file for the first time.

Move some unification related functions out of transfer into src/1

Document them in .doc files

Add type unification and mk_ucomb to boolSyntax

Add syntax/ops to make set manipulations easier

Put some syntax operations for bool$IN into boolSyntax.

Change src/bool to src/1 as a prelude to experimentation!