Resolve incorrect_partial_ord_impl_on_ord_type clippy lint

    warning: incorrect implementation of `partial_cmp` on an `Ord` type
       --> src/cxx_string.rs:242:1
        |
    242 | /  impl PartialOrd for CxxString {
    243 | |      fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        | | _____________________________________________________________-
    244 | ||         self.as_bytes().partial_cmp(other.as_bytes())
    245 | ||     }
        | ||_____- help: change this to: `{ Some(self.cmp(other)) }`
    246 | |  }
        | |__^
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incorrect_partial_ord_impl_on_ord_type
        = note: `-W clippy::incorrect-partial-ord-impl-on-ord-type` implied by `-W clippy::all`

src/cxx_string.rs

@@ -241,7 +241,7 @@ impl Eq for CxxString {}
 
 impl PartialOrd for CxxString {
     fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
-        self.as_bytes().partial_cmp(other.as_bytes())
+        Some(self.cmp(other))
     }
 }