Add sort support for inet, including support for abbreviated keys.
Testing has shown that this reduces the time taken to sort medium to
large inet/cidr inputs by ~50-60% in realistic cases.

Author: Brandur Leach
Reviewed-By: Peter Geoghegan, Edmund Horner
Discussion: https://postgr.es/m/CABR_9B-PQ8o2MZNJ88wo6r-NxW2EFG70M96Wmcgf99G6HUQ3sw@mail.gmail.com