client IPv6 support [BEP-7]

Depends on the next versions of the following modules:
* string2compat
* compat2string
* addr-to-ip-port

You can test with torrents from Bitlove.org, the tracker always returns the IPv4 & IPv6 address of the prittorrent seeder.
Oopsie. Ideally, we'd cover this with tests...