Fix issue #13352 - Only perform Algebraic arithmetic with allowed types.

Adds a static check allowed!T before trying any particular numeric type. Also checks for an exact type match in case of a raw (non-Variant) operand and prefers that to one of the fixed numeric types. Finally, it loosens the restriction for opArithmetic to only work for equal Variant types.

https://issues.dlang.org/show_bug.cgi?id=13352
Is this now unused?