Add camlp4 formula and hook it into OCaml/OPAM package

See #33596 for related discussion.
Camlp4 dynamically loads in syntax transformer libraries that parse an input AST (usually but not always OCaml) and then runs them through whatever's on the command line.

The standard path for finding these dynlinked libraries is embedded during compilation time.  It doesn't have to be, but there's ongoing work upstream to make this packaging more friendly now that it's out of the main compiler (e.g. ocaml/camlp4#51)

There's also an open bug report upstream for the OCaml compiler itself: http://caml.inria.fr/mantis/view.php?id=5950