[RFC] add program for returning a hash of an ocaml file

One way to solve [mantis 4991](http://caml.inria.fr/mantis/view.php?id=4991) (cmi partially written) is to create the output file at a temporary location (using -o), then move them to the real one, if it is different than the one already there.

This request for comment propose to add a new tool ocamlmd5sum, for computing a checksum of the .cmx, cmi, ... by reusing when possible the checksum already present in them.

Unfortunately it is not really faster than the venerable md5sum, but I don't understand why. I will also try to provide new ocamlbuild rules that use the method (temporary file) in order to show the advantage.