Implements keysSet

[kritzcreek]

Fixes #47

[hdgarrood]

It’s a bit odd to have this here as opposed to in maps, I think.

[kritzcreek]

@hdgarrood sets depends on maps, so it can't be in maps

[hdgarrood]

Right, sorry, I’m aware of that. I mean from the perspective of what API makes sense. I’m saying, perhaps we should consider reorganising things.

[kritzcreek]

I see, yeah I could see an ordered-containers package which contains all the structures depending on the ordered trie.

[hdgarrood]

See also #46. I think it makes sense to at least put maps and sets together, especially in light of this.