Aggregate takes a list of prefixes in conventional format on stdin,
and performs two optimizations to reduce the length of the prefix
list. It removes any supplied prefixes which are superfluous because
they are already included in another supplied prefix (e.g.,
18.104.22.168/24 would be removed if 22.214.171.124/17 was also supplied),
and identifies adjacent prefixes that can be combined under a single,
shorter-length prefix (e.g., 126.96.36.199/24 and 188.8.131.52/24 can be
combined into the single prefix 184.108.40.206/23).