Constant-time tag/digest validation against timing attacks, improved CMake portability.

Fixed

Security

• Use constant-time tag/digest validation to offer some resistance against
  timing attacks. Applies to all decryption functions
  (ascon_aead*_decrypt*()) and all hashing functions which compare
  the generated digest to the expected (ascon_hash_*_matches()).

Cross-platform portability

• Enforced name libascon.{dll|so|dylib} for the shared library, so it's the
  same when built with any toolchain.
• CMake will not verify whether a compiler supports a flag before using it.
  This makes the CMake configuration phase slightly longer, but the result
  is cached, so it happens only the first time.
• CMake now defaults to MinSizeRel build type if CMAKE_BUILD_TYPE is not
  specified, because binary size matters more than speed for a cross-platform
  implementation. Additionaly, on some platforms it overperforms the
  Release build.
• Explicitly setting many Doxygen settings.

Internal changes

• Simplified Ascon permutation code, to increase its readability.
• Internal function byte_mask() renamed to mask_most_signif_bytes().