Constant or constant functions

[Thomasdezeeuw]

Most types have function constructors, such as Domain::ipv4, should we make this functions const functions? Or alternatively make them associated constants?

impl Domain {
    /// Domain for IPv4 communication, corresponding to `AF_INET`.
    pub fn ipv4() -> Domain {
        Domain(sys::AF_INET)
    }

    /// Domain for IPv4 communication, corresponding to `AF_INET`.
    pub const IPV4: Domain = Domain(sys::AF_INET);
}

Personally I feel like associated constants would be more appropriate, as there constants wrapped in a type. But I don't feel strongly either way.

/cc @stjepang