Conditionally compile benches

The bench tests all need `rand` and one of `alloc`/`std`. Instead of
putting attributes everywhere just put one on the individual `benches`
modules.

src/ecdh.rs

@@ -235,7 +235,7 @@ mod tests {
     }
 }
 
-#[cfg(all(test, feature = "unstable"))]
+#[cfg(all(test, feature = "unstable", feature = "rand-std", any(features = "alloc", feature = "std")))]
 mod benches {
     use rand::thread_rng;
     use test::{Bencher, black_box};

src/ecdsa/recovery.rs

@@ -371,7 +371,7 @@ mod tests {
 }
 
 
-#[cfg(all(test, feature = "unstable"))]
+#[cfg(all(test, feature = "unstable", feature = "rand-std", any(features = "alloc", feature = "std")))]
 mod benches {
     use rand::{thread_rng, RngCore};
     use test::{Bencher, black_box};

src/lib.rs

@@ -984,7 +984,7 @@ mod tests {
     }
 }
 
-#[cfg(all(test, feature = "unstable"))]
+#[cfg(all(test, feature = "unstable", feature = "rand-std", any(features = "alloc", feature = "std")))]
 mod benches {
     use rand::{thread_rng, RngCore};
     use test::{Bencher, black_box};