[RFC] Implement LSP type/call hierarchies as actual hierarchies

[bstaletic]

This is the implementation of the server side changes needed to have useful support for LSP call/type hierarchies.
While the clien side changes deserve more attention, the server side should be fine to review and merge.

What's definitely missing are tests and API documentation.
On top of that, what do we want to do with the old GoToCollers/GoToCallees? I see three options:

• Remove them completely, which might break some users, but the new alternative should be better.
• Make them an alias for the new subcommands. This option wouldn't break users, but would break clients who would not adapt.
• Do nothing and maintain the old API forever, like with have done with other things in the past.

---

This change is 

[puremourning]

I need to schedule some time to play with this poc