Use test_hex_unwrap in bench code

We would like to move the dependency on `hex_lit` to be a
dev-dependency but currently are using it in bench code. The bench
code is enabled if any downstream crate tries to build with
`--cfg=bench` and during such a build our dev-dependencies are not
available.

We also have the `test_hex_unwrap` macro in the `hex` crate and since
the bench code is more or less test code (and the macro call is not
being benchmarked) we can use that macro instead.

bitcoin/src/blockdata/transaction.rs

@@ -2463,7 +2463,7 @@ mod tests {
 
 #[cfg(bench)]
 mod benches {
-    use hex_lit::hex;
+    use hex::test_hex_unwrap as hex;
     use io::sink;
     use test::{black_box, Bencher};