Operators should be defined as modules

[VonTum]

Instead of being a compiler builtin, our operators should be defined in a standard library .sus file

The naming scheme is still up in the air, but as an example:

module __add__ #(int MAX_A, int MAX_B) {
  interface __add__ : int #(MAX: MAX_A) a, int #(MAX: MAX_B) b -> int #(MAX: MAX_A+MAX_B)
  /* Implementation is a compiler built-in */
}

That way we also don't need to define the way these bounds evolve in the compiler itself, and thus the user can more easily see what's under the hood.