Dscanner should allow opCmp with no toHash

[JackStouffer]

Consider

struct S
{
        int opCmp(S rhs)
        {
            assert(0);
        }
}

void main()
{
        assert(S.init == S.init);
}

The absence of opEquals means it's using the language default, which means it's safe for S to use the default toHash as well.

dscanner should only throw a warning when opEquals exists but toHash doesn't.

[Hackerpilot]

I agree that this would be a good change.