provide constants for common (network) hosts to rama-net's Host

[GlenDC]

These constants can be added as associated constants to Host in

rama/rama-net/src/address/host.rs

Line 13 in c7f10e9

pub enum Host {

, e.g. as an impl block:

impl Host {
     /// description
     const NAME: Self = VALUE;

     /// ...
}

You can do so for the following list of common hosts:

Name	Value	Description
LOCALHOST_IPV4	127.0.0.1	Local loopback address (IPv4)
LOCALHOST_IPV6	::1	Local loopback address (IPv6)
LOCALHOST_NAME	localhost	Local loopback name
DEFAULT_IPV4	0.0.0.0	Default address, not routable
BROADCAST_IPV4	255.255.255.255	Broadcast address (IPv4)
EXAMPLE	example.com	Example domain for documentation

In case you know more commonly used Hosts do feel free to add some more.