[improve] Steroids for the type system

[nilq]

Wu should have sum types as well as interface-like type parameters. This will speed up the development process, while also making it even more fun and nice to use the language:

Cat: struct {
  weight: float
}

Dog: struct {
  hat: bool
}

dog_or_cat: Cat | Dog = new Cat {
  weight: 1000
}

# can later be Dog

Then the following:

Vector3: struct {
  x: float, y: float, z: float
}

Vector2: struct {
  x: float, y: float
}

move2D: fun(thing: { x: float, y: float}, dx: float, dy: float) {
  thing x += dx
  thing y += dy
}