Consolidate multiaddr transformations

[ghost]

We already support a number of map/filter like operations on multiaddrs, which take as input a list of multiaddrs and sometimes certain conditions, and return as output a list of multiaddrs.

• fmt (a.k.a. mafmt)
  • lets us define protocol schemata and filter addresses that match
  • e.g. the schema /ip6/udp matches /ip6/::1/udp/4002
  • implementations: go-multiaddr-fmt, js-mafmt
• filter
  • lets us filter addresses based on membership of an IP CIDR block
  • e.g. /ip4/192.168.0.0/ipcidr/16 matches /ip4/192.168.1.42
  • implementations: libp2p/go-maddr-filter
• addr-util
  • filters certain classes of IP addresses (link-local, transports that can't dial out, etc.)
  • implementations: libp2p/go-addr-util, go-multiaddr-net
• dns
  • resolves /dnsaddr, /dns4, and /dns6 addresses
  • implementations: go-multiaddr-dns, js-multiaddr

The first three should probably be collapsed into a single package.

A minimal generalized API for these transformations should be part of the multiaddr spec.

[ntninja]

@lgierth: Can you take a look at #87? Particularily wrt to the filter syntax it seems relevant, I believe.