message: crypto: speedup: replace stringstream by preallocated Buffers
  This is a resurection of a lost commit by Matthieu Nottale.
diff: @@ -57,11 +57,14 @@ namespace elle
  Mode const mode,
  Oneway const oneway) const
  {
  - auto _plain = elle::IOStream(plain.istreambuf());
  - std::stringstream _code;
  - this->encipher(_plain, _code,
  - cipher, mode, oneway);
  - return _code.str();
  + auto in = elle::IOStream(plain.istreambuf());
  + auto res = elle::Buffer();
  + res.capacity(plain.size() + 1024);
  + {
  + auto out = elle::IOStream(res.ostreambuf());
  + this->encipher(in, out, cipher, mode, oneway);
  + }
  + return res;
  }
  elle::Buffer
  @@ -70,11 +73,14 @@ namespace elle
  Mode const mode,
  Oneway const oneway) const
  {
  - auto _code = elle::IOStream(code.istreambuf());
  - std::stringstream _plain;
  - this->decipher(_code, _plain,
  - cipher, mode, oneway);
  - return _plain.str();
  + auto in = elle::IOStream(code.istreambuf());
  + auto res = elle::Buffer();
  + res.capacity(code.size());
  + {
  + auto out = elle::IOStream(res.ostreambuf());
  + this->decipher(in, out, cipher, mode, oneway);
  + }
  + return res;
  }
  void
  