Add simple Ip4Address/Ip4Network types

This patch adds some very simple IPv4 address/network types, and uses
them in the 'Network' config object.

We need these in order to properly compute the reserved IP addresses,
without depending on an external library (which I haven't found, by
the way). Currently the only operation supported is 'get next IP
address', which is enough for us.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>