Update Rust structure to better match proposed Python API

src/queue/keyed.rs

@@ -0,0 +1,25 @@
+// A "pure" priority queue that supports duplicates, but not arbitrary deletions or weight updates
+use crate::backing::{
+    item::Item,
+    pure::{binary_heap::BinaryHeap, PureBacking},
+};
+use pyo3::prelude::*;
+
+#[pyclass]
+pub struct KeyedQueue {
+    backing: Box<dyn PureBacking<Item<Py<PyAny>, f64>>>,
+}
+
+#[pymethods]
+impl KeyedQueue {
+    #[new]
+    fn new() -> Self {
+        Self {
+            backing: Box::new(BinaryHeap::new()),
+        }
+    }
+
+    fn __len__(self_: PyRef<'_, Self>) -> usize {
+        self_.backing.len()
+    }
+}