* src/lparse.ml (_lexp_decls): Rewrite to fix scoping issues

* src/lparse.ml (elab_check_sort, elab_check_proper_type): New functions.
(elab_check_def, ctx_define_rec._): Use them to check that the types
are proper types.
(lexp_detect_recursive, _lexp_rec_decl): Remove.
(lexp_check_decls, lexp_decls_1): New functions.
(_lexp_decls): Use them instead.

* src/opslexp.ml (check) <Arrow>: Fix incorrect context for target.

* src/util.ml (IntMap): New module.

* src/debug_util.ml (lexp_detect_recursive): Move from lparse.ml.