Add IpAddress-to-bytes conversion

This adds utilities aware of the validated format, making them correct
with respect to zone identifiers. The IPv4 conversion is also faster due
to the fact it can rely on well-formedness of the argument.

Change-Id: I28014d15107562e7dc6193eb6f8198586e839d5b
Signed-off-by: Robert Varga <rovarga@cisco.com>