Added a benchmark for set iter

benches/bench_set.rs

@@ -0,0 +1,21 @@
+#![feature(test)]
+
+extern crate test;
+use pyo3::prelude::*;
+use pyo3::types::PySet;
+use test::Bencher;
+
+#[bench]
+fn iter_set(b: &mut Bencher) {
+    let gil = Python::acquire_gil();
+    let py = gil.python();
+    const LEN: usize = 100_000;
+    let set = PySet::new(py, &(0..LEN).collect::<Vec<_>>()).unwrap();
+    let mut sum = 0;
+    b.iter(|| {
+        for x in set.iter() {
+            let i: u64 = x.extract().unwrap();
+            sum += i;
+        }
+    });
+}