Add address.get_canonical() and is_canonical()

These functions will take either an IPv4/IPv6 address, or an IPv4/IPv6
prefix, and cast them to their canonical form. Notably for IPv6 addresses,
this means lower case and with the 0-tuples correctly formatted:

2001:DB8::1 becomes 2001:db8::1
2001:db8:0:0::1 becomes 2001:db8::1

This avoids spurious diffs in vppcfg when comparing to the output of the VPP
dataplane.

vppcfg/config/loopback.py

@@ -83,6 +83,12 @@ def validate_loopbacks(yaml):
                         f"loopback {ifname} IP address {addr} conflicts with another"
                     )
                     result = False
+                if not address.is_canonical(addr):
+                    canonical = address.get_canonical(addr)
+                    msgs.append(
+                        f"loopback {ifname} IP address {addr} is not canonical, use {canonical}"
+                    )
+                    result = False
         if "mac" in iface and mac.is_multicast(iface["mac"]):
             msgs.append(
                 f"loopback {ifname} MAC address {iface['mac']} cannot be multicast"