Fix unused binding annotations

[ChrisPenner]

Overview

Previously I thought we wouldn't be able to get the correct locations of bindings for printing errors, but found we were actually stripping them during component minimization. This propagates the annotations so we can improve the location of unused bindings errors (and any other diagnostics which reference bindings in the future).

Old:

New:

Implementation notes

• Changes the annotations we have on Abs nodes to be the annotation of the actual variable
• Use the ann from the Abs node instead of the top-level-term
• Split to have a separate diagnostic for each var
• Set the diagnostics as "unnecessary" so they render as greyed-out in editors (just like HLS does it)

Interesting/controversial decisions

This does remove the invariant I had previously held where every node's annotations contains everything inside, but I think it's worth it to gain this functionality. I just special-cased the other commands to know to ignore the Abs annotation when looking things up.

Ideally I'd just have a spot to put a distinct annotation for variable bindings, but I can't do that without changing either the Term or the ABT which is a ton of work and would maybe affect hashing. Maybe we can address this when we do the next re-hash migration.

Test coverage

Added some clever tests where you can mark the expected diagnostic location for a given error.

Loose ends

The let-binding error still spans the entire binding rather than just the name. I think I can fix this by threading another annotation through the Unison File, but that's a bigger change for a pretty small value-add, so I'll leave that for later.

[ChrisPenner]

In the runtime we don't typically care about annotations, it's fine if we use the top-level annotation for arguments in lamdas (like we already do)