* src/typecheck.ml (check): Add missing cases (mostly, Case and Cons)

* src/lexp.ml: Move the subst helper functions and the unsusp here from
typecheck.ml.
(U): Use named import instead of `open` for Util.
(Case): Change pattern args so they always have an arg_kind.
(_lexp_to_str.arg_str): Change accordingly.

* src/lparse.ml: Remove all ";;": they are only meaningful to OCaml's
interactive loop and don't belong in source files.
(lexp_read_pattern, lexp_read_pattern_args): Adjust to new `Case` type.

* src/subst.ml (cons): New primitive.

* src/typecheck.ml: Move the subst helper functions and `unsusp` to lexp.ml.
(lookup_value): New function.
(check): Add missing cases (mostly, Case and Cons).