Counters of assertion passes and failures, simple status report printing,
check for non-all-zero arrays, fixes to avoid compilation warnings.

Added

• Counters atto_counter_assert_passes and atto_counter_assert_failures
  counting the amount of assertions atto_assert() or other aliases for it
  that failed or passed, to be able to see the progress through the test
  suite.
• atto_report() to provide a simple one-line indication of what is the
  status of the 2 counters above at any point in the test suite,
  including at the very end.
• atto_nzeros() which is the opposite of atto_zeros(), checking that
  there is at least one non-zero value in the array of bytes.
  Useful to check whether a memory location has been initialised to non-zero
  values, especially for random strings or data from an outside
  source we don't know the exact format of.
• GitHub workflow scripts to build and self-test on different operating
  systems. Fancy badges in the readme included.

Fixed

• CMake configuration now uses a more portable set of compiler flags,
  taken from LibAscon.
• Some macro arguments did not have round brackets around them.
• Self-test changed to avoid compilation warnings due to improper use of types
  (integers for float/double functions and signed/unsigned warnings).
• Cast the length values passed to atto_zeros() to size_t to avoid
  warnings about signed/unsigned integer comparison.
• Remove .idea folder from Git repo from now on.
• Fixed some typos/missing words in the BSD 3-Clause license text file.
• Renamed Doxygen build target to avoid name clashes.
• Fix typo in Doxygen config, now it should properly exclude the test
  and source file.
• Fixed self-test skipping some of the checks.