Compilation fixes for some GCC versions, added `ascon_` to some internal target names to avoid name clashing

Fixed

• Avoid type-punning when clearing the context and when comparing the
  expected and computed tag/digest, which may cause compilation errors
  with -Werror=strict-aliasing in some GCC versions.
• Renamed some build targets to avoid name collisions when including the whole
  LibAscon project into another CMake project with add_subdirectory()
  • doxygen -> ascon_doxygen
  • benchmark -> ascon_benchmark
  • copytestvectors -> ascon_copytestvectors
    As these are all internal targets, it should hopefully not affect the
    library user.