Add basic connectip client transport implementation

[dpeckett]

• Reuse the netstack tun device implementation from wireguard tunnel implementation.
• Massively simplify packet copying routines (sync.Pool isn't always needed or beneficial if we're immediately copying anyway).
• Add a muxed connect-ip connection type and use it in the server, use an old triemap library of mine instead of haxmap so that we can route properly based on containing prefixes. That library ought to become more popular :).