Applied iwyu --comment to the code base (#588)

include/gsl/pointers

@@ -17,11 +17,13 @@
 #ifndef GSL_POINTERS_H
 #define GSL_POINTERS_H
 
-#include <gsl/gsl_assert>
+#include <gsl/gsl_assert>  // for Ensures, Expects
 
-#include <iosfwd>
-#include <memory>
-#include <type_traits>
+#include <algorithm>    // for forward
+#include <iosfwd>       // for ptrdiff_t, nullptr_t, ostream, size_t
+#include <memory>       // for shared_ptr, unique_ptr
+#include <system_error> // for hash
+#include <type_traits>  // for enable_if_t, is_convertible, is_assignable
 
 #if defined(_MSC_VER) && _MSC_VER < 1910
 #pragma push_macro("constexpr")