Turned on Level 3 warnings for MSVC.

include/span.h

@@ -55,6 +55,7 @@
 // turn off some misguided warnings
 #pragma warning(push)
 #pragma warning(disable: 4351) // warns about newly introduced aggregate initializer behavior
+#pragma warning(disable: 4512) // warns that assignment op could not be generated
 
 #endif // _MSC_VER <= 1800