#6388: more invariant checks on the Parsetree. Reject 0- and 1-ary tuples, empty record (exprs, types, pats) and empty variant types.

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@14949 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02