Debug mode: learning to crawl.  I need to set up some tests that actually test that the debug mode is working, but that won't cause problems when debug mode isn't on.  This is my first prototype of such a test.  It should call std::terminate() because it's comparing iterators from different containers.  And std::terminate() is rigged up to exit normally.  If debug mode fails, and doesn't call terminate, then the program asserts.  The test is a no-op if _LIBCPP_DEBUG2 is not defined or is defined to be 0.