Add ByteIpAddr type serialize to byte representation

[canoriz]

In BitTorrent BEP10, extend handshake sends IPv6/IPv4 address in a "compact form", which is the big-endian byte representation, represented by byte string length of 4(ipv4)/16(ipv6). This commit adds a ByteIpAddr type to serialize and deserialize between core::IpAddr and byte string representation.

[canoriz]

I fixed the build error when enable alloc and disable std feature.

[bluk]

Thanks for the PR. The implementation seems fine and would be useful, but this functionality is better suited for a common BitTorrent library. There is nothing specific in the implementation requiring this bt_bencode library and in case you switch to a different bencode Serde based library, you could still re-use this type without pulling in this crate.