Get rid of unused `it` field in Case; various extra type checks

* src/lparse.ml (_lexp_p_check): Check the lambda's type annotations
are indeed types.
(lexp_case): Check that the subject is indeed of inductive type.
(lexp_call): Remove unused `from_lctx`.
(lexp_read_pattern): Remove unused `exp` arg.

* src/lexp.ml (lexp): Remove unused "base inductive type" field from `Case`.