feat(rlp): add encode for &str (#147)

* feat(rlp): add encode for &str

* add test

crates/common/rlp/src/encode.rs

@@ -291,6 +291,16 @@ mod alloc_support {
         }
     }
 }
+
+impl Encodable for &str {
+    fn encode(&self, out: &mut dyn BufMut) {
+        self.as_bytes().encode(out);
+    }
+    fn length(&self) -> usize {
+        self.as_bytes().length()
+    }
+}
+
 slice_impl!(Bytes);
 slice_impl!(BytesMut);
 
@@ -369,6 +379,13 @@ mod tests {
         out1
     }
 
+    #[test]
+    fn rlp_str() {
+        assert_eq!(encoded("")[..], hex!("80")[..]);
+        assert_eq!(encoded("{")[..], hex!("7b")[..]);
+        assert_eq!(encoded("test str")[..], hex!("887465737420737472")[..]);
+    }
+
     #[test]
     fn rlp_strings() {
         assert_eq!(encoded(hex!(""))[..], hex!("80")[..]);