ASCII->POLY: rm obsolete comments
PUB-API: rename headers to have ntru_ prefix
PUB-API: add missing const modifiers
ALL: rearrange out-parameters to consistently match flint logic
ERR: improve doxygen doc
DOC: add Hacking guide
ENC/DEC: implement compression via lz4

This speeds up encryption/decryption considerably, but also
causes complete failure for any non-recoverable char in a message,
since the decompression will fail then.

Also remove the double base64 encode/decode.