add (singleton) class IntegerRing
add FactorRing
major overhaul: switch to copy-on-write for Integer. Also remove the 'Ring' classes themselves (they were kinda cool but not that useful actually).

This COW system costs performance in a couple of cases (some of which can and should be circumvented at some point), but makes everything so much easier. In particular there are no more expensive postblits, so there is no need for ref-parameters anymore.