implement exhaustive switch completion

[zth]

This is a proposal to solve #253.
It's also the first step of #684, moving snippets into the analysis bin and only outputting them where they make sense.

(this GIF isn't 100% accurate, I've done some cosmetic fixes to the completion list since recording it)

This PR does the following: When completing..

• an ident after a switch
• and the switch has no cases
• and the type of the ident we're completing is something we can (reasonably) generate an exhaustive switch for
• ...add a separate completion item for the ident that, when chosen, inserts an exhaustive switch statement for that ident

Closes #253

[cristianoc]

This addition is pretty powerful.
Thoughts about the case where one already has a switch, with a catch-all | _ => and wants to complete the missing cases in place of _?

[zth]

I think that'd be a good addition. Perhaps a code action? Let's think a bit on it.

It's also related to filtering out "seen patterns" in pattern completion, which I'm going to tackle soon. As in | Some(true) | None = Some(false) is missing, rest is seen.

[amiralies]

Nice job!