Add small key derivation benchmarks

Cargo.toml

@@ -45,3 +45,7 @@ criterion = "0.3"
 
 [lib]
 bench = false
+
+[[bench]]
+name = "small"
+harness = false

benches/small.rs

@@ -0,0 +1,19 @@
+use criterion::{criterion_group, criterion_main, Criterion};
+use orchard::keys::{FullViewingKey, SpendingKey};
+
+fn key_derivation(c: &mut Criterion) {
+    // Meaningless random spending key.
+    let sk = SpendingKey::from_bytes([
+        0x2e, 0x0f, 0xd6, 0xc0, 0xed, 0x0b, 0xcf, 0xd8, 0x07, 0xf5, 0xdb, 0xff, 0x47, 0x4e, 0xdc,
+        0x78, 0x8c, 0xe0, 0x09, 0x30, 0x66, 0x10, 0x1e, 0x95, 0x82, 0x87, 0xb1, 0x00, 0x50, 0x9b,
+        0xf7, 0x9a,
+    ])
+    .unwrap();
+    let fvk = FullViewingKey::from(&sk);
+
+    c.bench_function("derive_fvk", |b| b.iter(|| FullViewingKey::from(&sk)));
+    c.bench_function("default_address", |b| b.iter(|| fvk.default_address()));
+}
+
+criterion_group!(benches, key_derivation);
+criterion_main!(benches);