Add sort support routine for the inet data type.

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

Author: petergeoghegan