add simple RedeemNode benchmark

We are going to change a bunch of stuff about type inference, so add a benchmark
for decoding a fixed program that's available. This is not the best benchmark,
but it's one that's available.

On my system the current results is: 70,972.00 ns/iter (+/- 662.39)

Author: apoelstra

src/node/redeem.rs

@@ -1102,3 +1102,39 @@ main := comp input comp process jet_verify : 1 -> 1"#;
         );
     }
 }
+
+#[cfg(bench)]
+#[cfg(feature = "elements")]
+mod benches {
+    use super::*;
+
+    use crate::bit_encoding::BitIter;
+    use crate::jet::Elements;
+
+    use test::{black_box, Bencher};
+
+    #[bench]
+    fn decode_fixed_program(bh: &mut Bencher) {
+        let prog = &[
+            0xd3, 0x69, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x3b,
+            0x78, 0xce, 0x56, 0x3f, 0x89, 0xa0, 0xed, 0x94, 0x14, 0xf5, 0xaa, 0x28, 0xad, 0x0d,
+            0x96, 0xd6, 0x79, 0x5f, 0x9c, 0x63, 0x47, 0x07, 0x02, 0xc0, 0xe2, 0x8d, 0x88, 0x10,
+        ][..];
+        let witness = &[
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x3b, 0x78, 0xce,
+            0x56, 0x3f, 0x89, 0xa0, 0xed, 0x94, 0x14, 0xf5, 0xaa, 0x28, 0xad, 0x0d, 0x96, 0xd6,
+            0x79, 0x5f, 0x9c, 0x63, 0xdb, 0x86, 0x8d, 0x45, 0xa0, 0xbc, 0x1d, 0x19, 0x01, 0x30,
+            0x2b, 0xc8, 0x7a, 0x87, 0x1c, 0xf1, 0x58, 0xe2, 0xbd, 0xe2, 0xcf, 0xa6, 0x45, 0xa8,
+            0x95, 0xc1, 0xb4, 0x5d, 0x68, 0xea, 0x24, 0xc0,
+        ][..];
+        bh.iter(|| {
+            let prog = BitIter::from(prog);
+            let witness = BitIter::from(witness);
+            black_box(RedeemNode::<Elements>::decode(
+                black_box(prog),
+                black_box(witness),
+            ))
+            .unwrap();
+        });
+    }
+}