src/array.php: Add function 'commonPrefix' along with relevant test-case.
src/string.php: Re-implement 'commonPrefix' using the function by the same name in the ArrayLib.
src/test/base-framework.php: Add 'TODO' item regarding removal of use of 'set_exception_handler'.
src/test/base-framework.php: Rework exception-handler to display more-readable stack trace.