Fix not(u64_digit) U32Digits::next_back for MSRV 1.31

Author: cuviper

src/biguint/iter.rs

@@ -156,7 +156,7 @@ impl Iterator for U32Digits<'_> {
 #[cfg(not(u64_digit))]
 impl DoubleEndedIterator for U32Digits<'_> {
     fn next_back(&mut self) -> Option<Self::Item> {
-        self.it.next_back().copied()
+        self.it.next_back().cloned()
     }
 }