Omp ocaml-migrate-parsetree-1.0

Driver: add --as-pp and --embed-errors flags.

--embed-errors causes the driver to embed exceptions raised by rewriters as extension points in the Ast

--as-pp is a shorthand for: --dump-ast --embed-errors

Expose more primitives for embedding the driver.

Fix bug where reset_args functions where not being called.

Fix "OCaml OCaml" in error messages (contributed by Adrien Guatto).