Support gentype

[jchavarri]

Mostly dumping what @anmonteiro and @andreypopp mentioned on Discord:

Approach 1 (conservative)

• port gentype to a melange-compatible typedtree
• implement dune rules to call gentype after compilation

Approach 2 (experimental)

Implement gentype as a deriving ppx that operates over mli/rei files. This approach wouldn't require typed tree nor custom dune rules, so in that sense it's simpler. But because the original gentype does rely on typed tree, it probably requires quite some more changes than the conservative approach.

[XantreDev]

port gentype to a melange-compatible typedtree

Are typedtrees significantly different?

[jchavarri]

I am not 100% sure but I suspect they must be quite similar. The largest differences will probably be caused by Melange supporting a much modern version of the OCaml compiler.