Improve AdicPrimitive::Natural and check through types on AdicPrimitive impl-ers

It would be nice to tighten up the Natural types on ZAdic, QAdic, and AdicPower. Technically we can make BigRational work for a lot of stuff. But we should prefer something more flexible and more accurately representing the relationship between AdicPrimitive and Natural.

Find a way to extend the bridge of PrimedFrom/Into and make it reversible:

• Natural -> AddPrime -> Primed
• Primed -> AddPrime -> Composite
• Primed -> DelPrime -> Natural

Find a way to share this Natural behavior with AdicComposite. Consider storing more in the type of AdicComposite, e.g. number of primes in the number.