Take advantage of new compiler features

[voodoos]

This is an issue to keep track of the work that remain to be done on Merlin to take advantages of recent compiler features:

In 5.4:

• [ ] https://github.com/ocaml/ocaml/pull/13302 and https://github.com/ocaml/ocaml/pull/13848 we could use it to provide correct occurrences and renaming of modules appearing in paths (and possibly other related features)

Unreleased:

In 5.5:

• [ ] https://github.com/ocaml/ocaml/pull/14120 this should finally enable us to provide correct renaming even in functors bodies.