Generalised the sum functions to monoidal folds, and re-implemented the sum instantiations, along with products.
Complex arithmetic via a new 'ComplexC' class. This uses, at the moment, TypeFamilies, though maybe the FunctionalDependencies variant is better.
Forgot to actually add the ComplexC module...
Complex numbers can now be proper math-shown, with correct rough-rounding of too-long finite or infinite decimal expansions.
Repaired some details about the unary minus operator and complex number comparison and math-rendering. Added test examples.