Move some middle-end files around (ocaml#2281)

* Various file moves in the middle end: this is the first stage of improving separation between the middle end and backend.
* Creation of file_formats/ directory (with associated file moves) to hold the definitions of compilation artifact formats.
* Creation of lambda/ directory (with associated file moves) to hold Lambda language definition files, transformation passes and construction passes from Typedtree.
* Disable (hopefully temporarily) dynlink, debugger and ocamldoc for the dune build.

Author: mshinwell

.depend

@@ -73,6 +73,9 @@ Working version
 - #2280: Don't make more Clambda constants after starting Cmmgen
   (Mark Shinwell, review by Vincent Laviron)
 
+- #2281: Move some middle-end files around
+  (Mark Shinwell)
+
 - #2283: Add [is_prefix] and [find_and_chop_longest_common_prefix] to
   [Misc.Stdlib.List]
   (Mark Shinwell, review by Alain Frisch and Stephen Dolan)