Port benchmarks to Criterion.

With Criterion the benchmarks work on stable Rust, too.

Change-Id: I0d460ffd265e82a21c625c41930dcce7521e34af
diff --git a/Cargo.toml b/Cargo.toml
index ac7eb33..a01cb34 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -10,6 +10,14 @@
 opt-level = 3
 debug = false
 
+[[bench]]
+name = "benchmarks"
+harness = false
+
+[dev-dependencies]
+criterion = "0.3.3"
+criterion_bencher_compat = "0.3.3"
+
 [build-dependencies]
 bindgen = "0.54.0"
 cc = "1.0.55"