simplify reduction via promote_op

[simonbyrne]

We use Base.promote_op in a lot of other places, so it would make sense to use it here as well. This would also make it easier to extend. e.g. JuliaMath/FixedPointNumbers.jl#100 would only require adding 2 lines instead of 6.

[martinholters]

I'm usually rather wary about using promote_op, but on a quick glance, these uses seem kosher.

[simonbyrne]

Yeah, it would be nice if promote_op had some guidance on when it is valid to use. But at least with this structure, it is possible to manually specify the behaviour for any particular function.

[JeffBezanson]

I can answer that one: it calls return_type, and so should never be used.