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:

• Store locations of longidents components ocaml#13302 and Index all paths components ocaml#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:

• [tooling] Associate Uids to items that don't have a concrete definition ocaml#14120 this should finally enable us to provide correct renaming even in functors bodies.