Changes from 1.21.1 to 1.21.2

• Add support for SHUFFLE_AVX2_ENABLED and SHUFFLE_SSE2_ENABLED
  even if AVX2 or SSE2 is not available. See PR #347. Thanks to
  Thomas VINCENT.

• Upgrade internal-complib zstd from 1.5.0 to 1.5.2. Thanks to
  Mark Kittisopikul.

• Many small code improvements, improved consistency and typo fixes.
  Thanks to Dimitri Papadopoulos Orfanos.

• New HIDE_SYMBOLS CMake option to control the symbols exposure.
  Default is ON. Thanks to Mariusz Zaborski.