Idea: Pow trait

[clarfonthey]

I've been working on a crate which works with runtime-level units, and I think that having a generic Pow trait would be useful.

Essentially, it would be very similar to the other unary op traits, namely:

pub trait Pow<RHS> {
    type Output;
    fn pow(self, rhs: RHS) -> Self::Output;
}

And in most cases, this would simply do exponentiation by squaring as included in the existing pow functions. However, it may be easier to implement for some types, namely a unit could simply multiply a field that indicates what power of the given unit you have. Another example is that a type which constrains which values it has, e.g. a matrix which specifically performs scaling operations, could be modified to do less work.

This would be weird to implement and may cause confusion with the existing pow function, which is why I haven't offered a PR for it. But it might be nice to have in this crate.

[cuviper]

It makes sense to me, especially since types can make internal decisions about stuff like #18.

[cuviper]

(We can de-emphasize the generic fn pow() in favor of this.)

[cuviper]

It might also be interesting that floats can implement both Pow<Self> and Pow<i32> using powf and powi, though only with std enabled. All of the integers can forward to their inherent pow methods too, instead of using our own.

[termoshtt]

Looks good :) I think it is very useful since it can be implemented for num_complex::Complex<T>.