Macro ipv6_addr_from_env_or

macro_rules! ipv6_addr_from_env_or {
    ($env_var:literal, $default:literal, $doc:literal $(,)?) => { ... };
}

Reads an IPv6 address at compile time from the given environment variable, produces an Ipv6Addr.

• The $default parameter allows to provide a fallback value for when the environment variable is not found.
• The $doc parameter allows to provide a documentation string for this tunable (see str_from_env!).

Produces a compile-time error when option_env! does.