use std in test configurations

this makes `cargo test` and `cargo bench` work cleanly, for one.

src/lib.rs

@@ -5,8 +5,13 @@
 #[allow(unstable)]
 extern crate core;
 
+#[cfg(test)]
+#[macro_use]
+extern crate std;
+
 pub use context::Context;
 
+#[cfg(not(test))]
 mod std { pub use core::*; }
 
 mod context;