Simplify the bind pattersn to single one in typed tree. Skeleton of compilation to lambda.

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/monadic_let@13391 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
More work on typing

git-svn-id: http://caml.inria.fr/svn/ocaml/branches/monadic_let@13392 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02