PPX distrib now can generate fully abstract types

Previously we needed to write to types in PPX

[%%distrib
  type ...t = ... fully_abstract ... [@@attrs optional]
  type ground = ...]

Now we can write only specialized ground. Another one will be generated automatically (like in noCanren). The old way is still being supported.

CC @dboulytchev @Lozov-Petr

TODO:

[ ] Mutual recursion is not supported (shouldn't be?)
[ ] The copy-paste from noCanren could be eliminated by putting extraction of fully abstracts to a separate lib, and using it from noCanren.

PLTools / OCanren