* src/lexp.ml (lexp_unparse): Prefer S.cons to S.Cons.

* src/subst.ml (subst): Add Myers's optimization in a comment.
It probably works, but it currently bumps into another performance problem.
(lookup, compose): Add commented-out Myers's optimization.

* tests/inverse_test.ml (is_identity): Remove (lets the code use the
version of Inverse_subst).