Use ocaml migrate parsetree and OCaml 4.08.1

[gaku-sei]

• [x] Updates dependencies
• [x] Use OCaml migrate parsetree to help parsing the AST
• [x] Use PPX Tools versioned to help writing the AST
• [x] Update code to be more "modern", using the last tools
• [x] Compiles to a self suffisent binary (no need for the --as-ppx trick anymore)
• [x] Supports officially and only @lenses and @deriving lenses
• [x] Supports all version of OCaml up to 4.08 (it used to be locked to 4.06)
• [x] Binaries are now smaller on both Darwin and Linux (~10mb)

🎊