Support for OCaml 4.13.0

[cphalen]

Between OCaml 4.10.0 (previous max supported version) and OCaml 4.13.0 there were two major changes:

• Stream was moved out of the stdlib into an opam package called camlp-streams, so I added the opam package to parts of the codebase that required Stream
• Parsetree was updated which required some changes:
  • Support for Pwith_modtype and Pwith_modtypesubst variants, these with constraints allow you to specify module type and module type substitutions
  • Support for Psig_modtypesubst variant which allows signatures to be defined with module type substitutions
  • Rearranging some type deconstruction for Ppat_construct which was restructured slightly

[Vighnesh-V]

lgtm chief ;)

[cphalen]

Code changes look good! Can you update CHANGELOG.md with the changes you made? Maybe bump version to 2.0.0 and also put that version into the opam and dune files?

Oh great point I'll do this today thanks Will