Port QArith

[sbp]

Whilst I only envisaged porting the P, N, and Z arithmetics from Coq, it might be nice to go for full-on completion and port QArith too. I'm not sure what the type should be, but probably something like this:

record Biq where
  constructor Ratio
  numerator: Biz
  denominator: Bip

The constructor could be Rational instead of Ratio, though by convention it should really be MkBiq. Coq uses num and den for the field names. I don't like "num", but perhaps n and d, or numer and denom would be okay.

[sbp]

Just to make things extra confusing, Coq also immediately defines a way to get the denominator as a positive Z, i.e. BizP in our nomenclature, which is then apparently used quite a lot.

[clayrat]

Sounds good, but we should probably finish P/N/Z first :)