* src/lparse.ml (_lexp_p_infer): Mostly cosmetic changes

* src/lparse.ml (elab_check_sort, elab_check_proper_type):
Make `var` into `option` type.
(lexp_type_infer): New function.
(_lexp_p_infer): Re-layout and indent.
Don't use OL.check to detect user errors.
Check that Phastype's annotation is a proper type.
(lexp_let_decls): Use fold_right.

* GNUmakefile (BUILDDIR): New var.  Use everywhere.