Add Lwt_cstruct.complete convenience function

When dealing with functions like 'read' and 'write' which return
an integer which means:
  0: End_of_file
  n: n bytes written
we can now write 'complete read' which will keep executing 'read'
until the whole buffer has been processed, returning () or
failing with End_of_file.

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge branch 'complete' of git://github.com/djs55/ocaml-cstruct into djs55-complete

Conflicts:
	CHANGES