Add support for absolute values

[droundy]

It would be lovely to be able to take the absolute value of a dimensioned number. I am not sure how to do this best, but would be happy to work on a pull request. Sadly, abs does not seem to be in a trait in the standard library, so we'd need specific implementations for basically every primitive type as far as I can see?

[paholg]

That would be great. Yeah, the only way I could see it is creating the trait and implementing it for all primitives.

You can see an example of this in Recip

dimensioned/src/traits.rs

Lines 156 to 166 in a946893

	pub trait Recip {
	/// The resulting type after taking the reciprocal
	type Output;
	/// The method for taking the reciprocal
	fn recip(self) -> Self::Output;
	}
	impl_unary!(f32, Recip, recip);
	impl_unary!(f64, Recip, recip);

Abs probably doesn't need the associated type Output. That is, unless we want to support types that change under absolute value. I can't think of what those would be outside of typenum, unless we wanted to do something like have i32::abs() return a u32, but that is probably not useful.

[paholg]

Closed by #41.