Generic complex numbers and algebras, basic implementation

[fredrik-johansson]

Adds a generic implementation of the algebra $R[i]$ where $i^2 = -1$.

This is very basic, missing things like:

• Several conversion to/from other types
• Square and nth roots, elementary and transcendental functions for complex numbers
• Polynomial and matrix multiplication
• Optimizations for specific base rings

Three examples of things this can be useful for are shown in the Python docstring:

• Arithmetic in $\mathbb{Q}(i)$ (since there is currently no fmpqi -- note however that this is unoptimized and either ca or nf_elem is currently a better choice for performance)
• Arithmetic in $(\mathbb{Z}[x])[i]$ (as an alternative to $(\mathbb{Z}[i])[x]$)
• Arithmetic in $\mathbb{\overline{Q}}$ using pairs of real algebraic numbers