Refactoring cfb.encrypt, fix twofish cipher

- Refactoring cfb.encrypt, using typed array for ciphertext, performance almost the same as before maybe slightly better
- Fixes https://github.com/openpgpjs/openpgpjs/issues/158, https://github.com/openpgpjs/openpgpjs/issues/159 and https://github.com/openpgpjs/openpgpjs/issues/177
- Fix bug in twofish cipher which was broken due to typed array usage in cfb