Consider softening the demands on ComplexField

If I understand correctly every `RealField` must implement `ComplexField` (with the imaginary part set to zero) and every `ComplexField` must be a super-set of `f64`, that is, it must be able to represent all values of f64 logically (disregarding machine specific limitations). [Decorum](https://github.com/olson-sean-k/decorum)'s `N64` and `R64` types logically cannot be NotANumber or Infinity. They thus are not logical super-sets of `f64`.

For my calculations results of NaN or Infinity are logical errors. I put finite numbers in and expect my calculations to result in finite numbers. I'd thus like to use Deorum's numbers.

My attempt to implement the traits, required for ComplexField, failed at the above requirement. Would it be possible to remove the requirement or replace it by a softer requirement, like Decorum's `Real` or `Real+Encoding`?