New null and lz4 compression.
Giving -C 0 now uses null compression, not zlib level 0 any more
(null has almost zero overhead while zlib-level0 still had to package everything into zlib frames).
Giving -C 10 uses new lz4 compression, super fast compression and even faster decompression.
See borg create --help (and --compression argument).
fix some issues, clean up, optimize:
CNULL: always return bytes
LZ4: deal with getting memoryviews
Compressor: give bytes to detect(), avoid memoryviews
for lz4, always use same COMPR_BUFFER, avoid memory management costs.
check --chunker-params CHUNK_MAX_EXP upper limit