Fix: add missing use statements

Author: wawwior

src/uint/boxed/encoding/serde.rs

@@ -24,6 +24,8 @@ impl Serialize for BoxedUint {
 #[cfg(test)]
 mod tests {
     use super::BoxedUint;
+    use crate::Limb;
+    use hex_literal::hex;
 
     #[test]
     fn serde_roundabout() {