add ecapnp_serialize:pack/1

The erlang version of the addressbook example now works as a drop-in replacement for the c++ version.
I.e.
$ .../ecapnp/priv/samples/addressbook.sh write | .../capnproto/c++/samples/addressbook read
Alice: alice@example.com
  mobile phone: 555-1212
  student at: MIT
Bob: bob@example.com
  home phone: 555-4567
  work phone: 555-7654
  unemployed