Add type conversions for {Hash,BTree}Sets

[mmirate]

The Lua convention for a set (S∋x¹ ^ S∋x² ^ ...) is that S is a table where each x is a key whose value is true.

The FromLua and ToLua implementations I'm proposing here, then. abstract-away the .iter().cloned().map(|k| (k, true)) dance needed to expose to Lua one of the Set types from std::collections.

[jonas-schievink]

IIRC those were removed intentionally after the discussion in #11

[mmirate]

Ah, interesting. Looks like the old behavior effectively constructed/deconstructed sets from vectors, and the old behavior's removal was due to the perception that the behavior in this PR was merely another valid way to interpret a std::collections Set.

I disagree, in that this behavior seems far more obvious.

... Though the conversion between a Set and a Map actually ends up doing more work than reconstructing a set-ish table from a Vec of the members (because iterating through a set or map is much less trivial than through a Vec). Quite annoying that there isn't a lower-cost way in std::collections to transform a Set + value-function to a Map nor a Map's keys to a Set; but without it, the abstraction I'm proposing does no good. (Nor does, really, using a std::collections Set with rlua in the first place.) sigh. Closing.