create Abs trait for absolute value

[droundy]

I wrote this very quickly, so please do check that I haven't missed anything. It's just got the one example as a test.

[paholg]

Looks good.

We could also add an impl for i128, but that was only fairly recently stabilized, so it's probably best to leave off for now if there's no demand for it.

It also occurs to me that we could do special case implementations for this and other functions on primitives, so they can be called without the traits in scope.

For example,

impl<U> $System<i32, U> {
    fn abs(self) -> Self { $System::new(self.value_unsafe.abs()) }
}

But that can be left for another time.

[droundy]

Do you do special case implementations like that for any other methods? It seems potentially very appealing, but I'd think that consistency would be key.

[paholg]

I don't think so. I think it just hadn't occurred to me before.

I think it would be worth adding for at least anything we have a trait for, as well as any other methods that exists on primitives.

I would happily accept such a PR, or may make one myself at some point.