Tweak use of substitutions and unsusp

* src/inverse_subst.ml (mkFinalShift): Remove; use S.shift instead.
(toIr): Use S.mkShift.
(flattenIr.flattenCons): Remove unused `offset' arg.  Simplify.

* src/lexp.ml (subst): Newtype.
(mkSusp): Also apply eagerly to Metavar.
(push_susp): Rename from unsusp.
(nosusp): Rename from ususp_all.
(_lexp_to_str): Use push_susp.

* src/unification.ml (unify.tmp): Use nosusp.
(_unify_susp): Remove.

* tests/inverse_test.ml: Use S.shift/cons and friends.