Add plumbing to integrate with ppx support in trunk ocamlfind

There's a pinnable repo of ocamlfind at https://github.com/whitequark/ocaml-findlib.

How it works:
```
# #require "ppx_tools";;
# [%expr 2+2];;
- : Parsetree.expression =
{Parsetree.pexp_desc = ...
```