chore(developer): further filesystem cleanup in kmcmplib

* fixes CheckFilenameConsistency to use Windows functions on Windows,
  to check case differences, and only test for file existence on other
  platforms which are already case sensitive.
* fixes memory leaks in usage of `wstrtostr2`, and eliminated the
  function altogether.
* avoided use of u16sprintf for error messages, as it seems not to be
  working correctly (long term, eliminate altogether, use C++ string
  formatting).
* fix memory leak and overwrite of token in compiler error message.
* remove debug logging and namespace from filesystem.cpp

Unrelated to kmcmplib:
* improved logs in kmcomp tests (these tests are running on kmcmplib),
  so it is easier to diagnose failing tests.