Investigate ocaml-migrate-parsetree

[IgnoredAmbience]

https://github.com/ocaml-ppx/ocaml-migrate-parsetree is a tool to enable ocaml tools to be more version independent of the ocaml compiler's ast structure. This would be ideal to integrate with our project.

[IgnoredAmbience]

We directly import use Typedtree (and other compiler internals) in js_of_ast.ml.

ocaml-migrate-parsetree is only capable of migrating Parsetrees and related types between compiler versions, it is therefore unlikely to be useful for migrating the main generator code. It may still be useful for our ppx rewriters.