New hashing functions, any tag/digest sizes, less dependencies, assertion checks for debug mode

New hashing functions comparing the expected and computed digest,
support for virtually any tag and digest size, removed dependencies malloc.h
and string.h, better context cleanup, optional asserts validating the
library input.

Added

• 4 new hash functions that compute the digest of a message and compare it with
  the expected one, informing the user if they match. This is done with
  constant stack memory usage, like the AEAD functions now validate the tags
  too. The functions need the data and the expected digest as input,
  providing a boolean as output, indicating whether the digest matches or not.

  • ascon_hash_matches() and ascon_hash_xof_matches() to validate the
    digest of a contiguous message.
  • ascon_hash_final_matches() and ascon_hash_xof_final_matches() to
    validate the digest as a last step of the Init-Update-Final process,
    removing the need from the user to call ascon_hash_final() or
    ascon_hash_xof_final() and run memcmp() on the just calculated digest.

• Optional runtime asserts to validate the argument of the
  library API functions, mostly checking for NULL pointers and correct
  order of calling of the Init-Update-Final functions.

  • Suggested use only in Debug mode.
  • Uses assert.h by default, but can be overridden by defining
    ASCON_ASSERT at compile time too.
  • In CMake script it's enabled only for Debug builds and only
    if assert.h is available.
  • Disabled by default if compiling the library by other means
    (e.g. custom makefile).

• 2 new example usages in the Readme:

  • Offline encryption/decryption of contiguous data.
  • Hashing functions, including new digest-comparison functions.

Changed

• Library internals (not impacting API):
  • The AEAD tag validation is not performed one chunk of 8 bytes at the time
    rather than generating the whole contiguous tag from the user-given data and
    comparing it in its entirety (memcmp()) with the user-given tag.
    This implies that tag lengths don't have a physical limitation anymore
    (previously tag lengths > 64 bytes were discouraged).
  • Renamed state variable ascon_bufstate_t.assoc_data_state
    to ascon_bufstate_t.flow_state.
  • Enlarged state enum ascon_flow_t.
  • Renamed const uint8_t* tag parameter in AEAD function to expected_tag
    to emphasise that is the one that comes next to the ciphertext.
    It's length is now similarly expected_tag_len.

Removed

• Dependency malloc.h: is not required on Windows anymore, as we don't
  allocate the whole expected tag on the stack anymore: a small 8 byte buffer
  is used instead.

• Dependency string.h: due to memcmp() (see Changed section) and
  memset()/memset_s() (see Fixed section) not being used anymore, the
  library is not used.

Fixed

• The clearing of the context, both for AEAD and hash functions is performed
  without loops or memset()/memset_s(), but by setting the (not so many)
  context fields one by one to 0 using volatile pointer dereferencing to
  improve the chances of the optimiser not removing the cleanup section.

• CMake fixes:

  • clean target now removes ascon.h from the build directory.
  • Better copying of the test vectors to the build directory: use a custom
    target, set it as a dependency to testascon and testasconshared to
    avoid issues on some systems.
  • Building with CMake should now work properly when using LibAscon in a
    Git Submodule.

• Small fixes in the hash/XOF function tests.