Defining methods on a class when a generic is a specific type

[cooper]

This is sorta an extension of #18.

class List <T=Str>

method join {
    need $by: Str
    $c = ""
    for $str in *self {
        $c += $str
    }
    return $c
}

This is a bad example because .join() exists for all types since all types are stringifiable.

[cooper]

This will probably never happen. Not sure it'd actually be useful.