Allow Result<IntoLuaMulti>'s to be used with IntoLuaMulti #710

Right now, a Result can be used as a IntoLuaMulti when it has IntoLua. This follows the convention of lua errors. However, it's not possible to return multiple values when using this structure. For example, a Result<(int, String)> cannot be used as a return value.

I would expect that the result would just return the passed value. Not sure if it makes sense to have multi value errors.

I know I can return a MultiValue directly, however, with the help of a utilitiy function, I have it where my function returns the Result that is parsed directly, so I use the ? operator a few spots that would make my code much longer if I used a multivalue directly.