Add support for SCION addresses

[leonrinkel]

We'd like to upstream libp2p support for the SCION^1 Internet architecture.

This PR adds support for multiaddrs like /scion/19-ffaa:1:1079. SCION IAs consist of a 16-bit ISD and a 48-bit AS number separated by -. ISD numbers are formatted as decimal (19), AS numbering is similar to IPv6 (ffaa:1:1079). ^3 A SCION multiaddr can encapsulate an IP address to specify an endpoint, e.g. /scion/19-ffaa:1:1079/ip4/127.0.0.1.

I have already reserved a corresponding protocol identifier in multiformats/multicodec#325. There also is an experimental go-libp2p SCION+QUIC transport available at netsys-lab/go-libp2p@7ea953ad7fed3715b5c9cc1ae4e12e381425c87d.