Defined parameters for specialization of polymorphic functions

[Victorious3]

When overloading a generic function that takes an argument of any type,
it is trivial to specialize it for a specific case by simply using that type for an argument:

def polymorph(a: type T) -> T {
    return a
}

def polymorph(a: int) -> int {
    return 20
}

However, for type parameters this is currently impossible.

It could look like this:

def polymporph(type T) -> int {
    return size_of T
}

def polymporph(int) -> int {
    return 20
}

Note that this doesn't conflict with default parameters, those would use an =