 a decompression function.
  Decompression functions only accepts one block at a time.
  The last 64KB of previously decoded data *must* remain available and unmodified at the memory position where they were decoded.
  If less than 64KB of data has been decoded, all the data must be present.

  Special : if decompression side sets a ring buffer, it must respect one of the following conditions :
  - Decompression buffer size is _at least_ LZ4_decoderRingBufferSize(maxBlockSize).
    maxBlockSize is the maxim