Improve isEmpty

[crgwbr]

• Make isEmpty work for Sets (Make isEmpty work for Sets #31)
• Make isEmpty a type guard (Make isEmpty a type guard #16)

[crgwbr]

Ping @yokuze

[onebytegone]

@crgwbr Sorry for the delay on this PR. By any chance could you create separate PRs for a couple of the changes in this? One for the isBoolean and isSet and another for the internal change from any to unknown on the other type guards? We can likely get those smaller PRs merged within a few days, which will reduce the testing load for this PR.

[crgwbr]

@onebytegone I've created #56 for the change from any to unknown and changed this PR to just by the changes to isEmpty. Couldn't quite to what you asked for though: isBoolean is required for #56, so that had to go there rather than here.

I suppose I could create 3 PRs:

1. Add isBoolean and isSet.
2. Use unknown types (would depend on 1).
3. Improve isEmpty (would also depend on 1).

Let me know what you want done.

[crgwbr]

Also, as two PRs now, there's going to be a merge conflict in src/utils/is-empty.ts for whichever of these PRs gets merged second. Let me know and I'll rebase it for you.

[onebytegone]

@crgwbr Could you split the isBoolean and isSet change into their own PR? Those can be merged with little risk and will resolve #49. The s/any/unknown/ changes will need more testing (tangentially, looking at #56, can we keep all the isEmpty changes in this PR).

[crgwbr]

@onebytegone Here ya go:

#57

#56

This MR (#34) is based on #57 and will need rebased once that's merged.