Takes a list of prefixes in conventional format on stdin, and performs two
optimisations to attempt to reduce the length of the prefix list.

The first optimisation is to remove any supplied prefixes which are
superfluous because they are already included in another supplied prefix.
For example, 203.97.2.0/24 would be removed if 203.97.0.0/17 was also
supplied.

The second optimisation identifies adjacent prefixes that can be combined
under a single, shorter-length prefix. For example, 203.97.2.0/24 and
203.97.3.0/24 can be combined into the single prefix 203.97.2.0/23.
