Gaussian rationals `fmpqi`

[rburing]

FLINT has an implementation of Gaussian integers in fmpzi, but not Gaussian rationals which would presumably be called fmpqi .

Consider e.g. a differential operator with rational polynomial coefficients, and wanting a basis of solutions at a given Gaussian rational; then one might want to shift the operator, etc.

[fredrik-johansson]

I think I will do this later today.

It's worth noting that nf_elem is already optimized for $\mathbb{Q}(i)$ but I agree having at least a basic standalone interface for fmpqi would be nice!