Move benchmarks under benches/ so tests compile on stable

Closes #36.

benches/pairing_benches.rs

@@ -0,0 +1,21 @@
+// `clippy` is a code linting tool for improving code quality by catching
+// common mistakes or strange code patterns. If the `clippy` feature is
+// provided, it is enabled and all compiler warnings are prohibited.
+#![cfg_attr(feature = "clippy", deny(warnings))]
+#![cfg_attr(feature = "clippy", feature(plugin))]
+#![cfg_attr(feature = "clippy", plugin(clippy))]
+#![cfg_attr(feature = "clippy", allow(inline_always))]
+#![cfg_attr(feature = "clippy", allow(too_many_arguments))]
+#![cfg_attr(feature = "clippy", allow(unreadable_literal))]
+
+// The compiler provides `test` (on nightly) for benchmarking tools, but
+// it's hidden behind a feature flag. Enable it if we're testing.
+#![cfg_attr(test, feature(test))]
+#[cfg(test)]
+extern crate test;
+
+extern crate rand;
+
+extern crate pairing;
+
+mod bls12_381;