renaming main logic files. Added warning message of the removal and include passthrough. Renamed includes in the source files. Ran Clang-Format (#946)

Header rename

tests/span_tests.cpp

@@ -16,8 +16,8 @@
 
 #include <gtest/gtest.h>
 
-#include <gsl/gsl_byte> // for byte
-#include <gsl/gsl_util> // for narrow_cast, at
+#include <gsl/byte> // for byte
+#include <gsl/util> // for narrow_cast, at
 #include <gsl/span>     // for span, span_iterator, operator==, operator!=
 
 #include <array>       // for array