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
It's not ok for const exceptions... by virtue of their relationship, all problems with immutable -> mutable are shared by const -> mutable:
```d
static immutable ex = new Exception("");
const constEx = ex;
bool ran = false;
try throw constEx;
catch(Exception e)
    ran = true;
assert(ran);
```
> I'd say we are in desperate need of reused exceptions in Phobos

It's not as simple as it's done here. We need to make sure the chaining mechanism supports it in a reasonable manner.