Implement Args for Vec

[MinusGix]

The first commit changes the Args::count method to take a self parameter, as the size for a Vector can not be known at compile-time like the size of tuples. Hopefully, such a change won't cause worse optimization for tuples as all Args are used as generic parameters and so are known what they are compile-time.

The second commit implements Args for a Vec<runestick::Value>, as well as adding an example of it being used. The use-case for this is when you want to pass parameters of an unknown amount into a rune function.
This could be changed to be more generic instead, such as implementing Args for Vec<T> where T: ToValue. That would include Value, and other Vectors of types which could be transformed into values. If you'd prefer, I could change the pr to implement it for T: ToValue.

[udoprog]

This is perfect. Much appreciated!