Add a stub Rust crate (#12595)

rust/src/lib.rs

@@ -0,0 +1,15 @@
+use pyo3::prelude::*;
+
+/// Formats the sum of two numbers as string.
+#[pyfunction]
+#[pyo3(text_signature = "(a, b, /)")]
+fn sum_as_string(a: usize, b: usize) -> PyResult<String> {
+    Ok((a + b).to_string())
+}
+
+/// The entry point for defining the Python module.
+#[pymodule]
+fn synapse_rust(_py: Python<'_>, m: &PyModule) -> PyResult<()> {
+    m.add_function(wrap_pyfunction!(sum_as_string, m)?)?;
+    Ok(())
+}