Refactor

[nicholaschiasson]

Clean up code a bit, for example, expose an Fcidr error type (don't just use the Cidr error type everywhere).

Also investigate ways to improve error handling, removing impossible error scenarios from the code so that not everything needs to return a result. For example, Fcidr::new should not need to return a result, and frankly, neither should Fcidr::include or Fcidr::exclude.