full variant support

[belm0]

Without this it isn't possible for a Lua function to return variable number of arguments of differing type (see issue #42).

(This may not be high priority but I want to document it.)

[JakobOvrum]

Yes, it is still possible in various different ways, as explained in #42.

I am waiting for the std.variant rewrite before implementing Variant; it is possible with the current state of std.variant too, but it's not practically useful because pushing one to Lua would have awful algorithmic complexity.