[Fix #776, #773, #733] Rewrite bencode decoder

  - nREPL dicts are now plists and accessor api is given by `nrepl-dict-p`,
    `nrepl-dict-get` and `nrepl-dict-put`.
  - nested stack is used for decoded messages to avoid re-parsing of incomplete messages
  - queues are used for raw strings from server and for decoded requests