no_std support

[rye]

While this crate does fit the use case I have for it, (that is, parsing and manipulating crates in non-embedded use cases) it'd be nice to add no_std support in case someone wants to use this crate.

The main barrier here is that IpAddr is not part of core; it is in std::net.

Ipv4Addr and Ipv6Addr, unfortunately, are apparently wrappers over the system in and in6 things…, which unfortunately means it'll be non-trivial to port away.

I might need to implement Ip.*Addr things myself eventually.