Support passing Results to Lua

[jonas-schievink]

We already have a ToLua impl for Option, which will push the contained value if it exists, and nil otherwise. This maps pretty well into Lua's world.

Another common Lua pattern occurs when dealing with fallible operations like io.open, which either returns the opened file on success, or nil followed by an error message on failure. It would be nice if I could map Rust's Result to this idiom. This would require a ToLuaMulti impl instead of a ToLua impl, though (since the operation can push nil and the error message).