bugfix: fixes not operator creates invalid types

* fixes Not implementations creating types with
  out-of-bounds representations

Author: Ben Leadbetter

Diff for: src/lib.rs

@@ -970,4 +970,9 @@ mod tests {
         assert_eq!(!u7(0), u7(0x7F));
         assert_eq!(!u7(56), u7(71));
     }
+
+    #[test]
+    fn test_not_creates_valid_repr() {
+        assert_eq!((!u4(0b0101)).0, 0b1010);
+    }
 }