Remove unused headers from gsl/pointers

forward is already declared in utility, no need to include algorithm which is relativaly heavy
hash is already declared in memory, no need to bring brand-new header system_error for hash only

include/gsl/pointers

@@ -19,12 +19,10 @@
 
 #include "assert" // for Ensures, Expects
 
-#include <algorithm>    // for forward
 #include <cstddef>      // for ptrdiff_t, nullptr_t, size_t
-#include <memory>       // for shared_ptr, unique_ptr
-#include <system_error> // for hash
+#include <memory>       // for shared_ptr, unique_ptr, for hash
 #include <type_traits>  // for enable_if_t, is_convertible, is_assignable
-#include <utility>      // for declval
+#include <utility>      // for declval, for forward
 
 #if !defined(GSL_NO_IOSTREAMS)
 #include <iosfwd> // for ostream