Add a new "normalize" function to the Cidr module.

Add a new "normalize" function to the Cidr module.

There are several different ways to represent the same Cidr, as their
Eq instance shows. The canonical way to represent a given CIDR is with
its host bits zeroed out; for example, "127.0.0.0/8" instead of
"127.0.0.1/8". This commit adds a normalization function to the Cidr
module.

It should be possible to use the new function to clean up the code
some, and to allow the user to specify normalized output for the
"reduced" mode.