Merge parsing of class declaration / class definition. (This introduces conflicts in the grammar, I don't understamd why at this point.)

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/merge_sig_str@15628 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Allow let binding in signatures, syntactically.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/merge_sig_str@15629 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02