[Proposal] Make Numeric Refine a new AdditiveArithmetic Protocol

[rxwei]

Pitch thread: Make Numeric Refine a new AdditiveArithmetic Protocol

[natecook1000]

I meant to note that this isn't ambiguous, since the type checker will always first try to use a literal's default type (Int for integer literals). If there's a use that supports that, the default wins. When there's nothing available using the default, then there's an ambiguity, so making this example use a Vector<Double> would be a better illustration. There's no precedence in that case between Vector<Double> and Vector<Double>.Scalar (i.e. Double), so there's an ambiguity.