A PPX Rewriter for Migrating AST types (written using Camlp5)

This is a PPX Rewriter for generating "migrations" like those written by-hand (with some automated support) in "ocaml-migrate-parsetree". The goal here is that the input to the automated tool is the minimum possible, with no need for human massaging of output from the tool.

There are two examples: a small one (in a unit-test) and a full set of migrations from Ocaml's AST 4.02 all the way up to 5.2.0, with all the intermediate ASTs, and migrations forward as well as backward.