checker, cgen: allow op overload for type with generic parent

[Delta456]

This PR compiles the following code:

import math.vec


type Vec3 = vec.Vec3[f64]

fn main() {
	a := Vec3{1.0, 2.0, 3.0}
	b := Vec3{0.5, 1.5, 2.5}
	println(a - b)
}

Previously it wasn't working.

[spytheman]

Excellent work.