Update Rust structure to better match proposed Python API

src/lib.rs

@@ -2,11 +2,11 @@ pub mod backing;
 pub mod queue;
 
 use pyo3::prelude::*;
-use queue::pure::PureQueue;
+use queue::keyed::KeyedQueue;
 
 /// Bindings for the Rust queue implementations
 #[pymodule]
 fn pyority_queue(m: &Bound<'_, PyModule>) -> PyResult<()> {
-    m.add_class::<PureQueue>()?;
+    m.add_class::<KeyedQueue>()?;
     Ok(())
 }