Powerset: add benchmarks

Add benchmarks for Combinations iterator for combination lengths of 1,
2, 3, 4 and 14.

Renames tuple_combinations test functions to tuple_comb_* for clarity
in test results.

Cargo.toml

@@ -71,3 +71,7 @@ harness = false
 [[bench]]
 name = "combinations"
 harness = false
+
+[[bench]]
+name = "powerset"
+harness = false

benches/powerset.rs

@@ -0,0 +1,77 @@
+use criterion::{black_box, criterion_group, criterion_main, Criterion};
+use itertools::Itertools;
+
+// Keep aggregate generated elements the same, regardless of powerset length.
+const TOTAL_ELEMENTS: usize = 1 << 16;
+const fn calc_iters(n: usize) -> usize {
+    TOTAL_ELEMENTS / (1 << n)
+}
+
+fn powerset_n(c: &mut Criterion, n: usize) {
+    let id = format!("powerset {}", n);
+    c.bench_function(id.as_str(), move |b| {
+        b.iter(|| {
+            for _ in 0..calc_iters(n) {
+                for elt in (0..n).powerset() {
+                    black_box(elt);
+                }
+            }
+        })
+    });
+}
+
+fn powerset_0(c: &mut Criterion) { powerset_n(c, 0); }
+
+fn powerset_1(c: &mut Criterion) { powerset_n(c, 1); }
+
+fn powerset_2(c: &mut Criterion) { powerset_n(c, 2); }
+
+fn powerset_3(c: &mut Criterion) { powerset_n(c, 3); }
+
+fn powerset_4(c: &mut Criterion) { powerset_n(c, 4); }
+
+fn powerset_5(c: &mut Criterion) { powerset_n(c, 5); }
+
+fn powerset_6(c: &mut Criterion) { powerset_n(c, 6); }
+
+fn powerset_7(c: &mut Criterion) { powerset_n(c, 7); }
+
+fn powerset_8(c: &mut Criterion) { powerset_n(c, 8); }
+
+fn powerset_9(c: &mut Criterion) { powerset_n(c, 9); }
+
+fn powerset_10(c: &mut Criterion) { powerset_n(c, 10); }
+
+fn powerset_11(c: &mut Criterion) { powerset_n(c, 11); }
+
+fn powerset_12(c: &mut Criterion) { powerset_n(c, 12); }
+
+fn powerset_13(c: &mut Criterion) { powerset_n(c, 13); }
+
+fn powerset_14(c: &mut Criterion) { powerset_n(c, 14); }
+
+fn powerset_15(c: &mut Criterion) { powerset_n(c, 15); }
+
+fn powerset_16(c: &mut Criterion) { powerset_n(c, 16); }
+
+criterion_group!(
+    benches,
+    powerset_0,
+    powerset_1,
+    powerset_2,
+    powerset_3,
+    powerset_4,
+    powerset_5,
+    powerset_6,
+    powerset_7,
+    powerset_8,
+    powerset_9,
+    powerset_10,
+    powerset_11,
+    powerset_12,
+    powerset_13,
+    powerset_14,
+    powerset_15,
+    powerset_16,
+);
+criterion_main!(benches);