Easy conversions between Signed and Unsigned

[clarfonthey]

It would be nice if Signed and Unsigned had a way to convert between each other, presumably via:

• Signed::abs_unsigned, which would losslessly convert the signed version to an unsigned version of the same size (wrapping_abs + as for primitives, and just extracting the inner BigUint for BigInt)
• Unsigned::try_signed, which would allow casting to the signed version if available.
• Signed::Unsigned and Unsigned::Signed, two associated types that would do the job.

This would be a breaking change but a useful one. For example, a Pow implementation for Ratio could leverage these methods so that the signed and unsigned versions relied on each others' implementations, instead of duplicating code.

[cuviper]

I've decided to limit the clutter of breaking change issues, so they will now be tracked collectively in #47. I'm closing this issue for now as postponed but we can still discuss its merits here as needed.