Introduce "benches" feature

So that benchmarks don't run each time tests run

Cargo.toml

@@ -36,3 +36,6 @@ ring = { git = "https://github.com/paritytech/ring" }
 [[bin]]
 path = "lighthouse/main.rs"
 name = "lighthouse"
+
+[features]
+benches = []

lighthouse/state/block/validation/benches.rs

@@ -65,7 +65,6 @@ fn bench_block_validation_scenario<F>(
 }
 
 #[bench]
-#[ignore]
 fn bench_block_validation_10m_eth(b: &mut Bencher) {
     let total_validators: usize = 10_000_000 / 32;
     let cycle_length: u8 = 64;
@@ -100,7 +99,6 @@ fn bench_block_validation_10m_eth(b: &mut Bencher) {
 }
 
 #[bench]
-#[ignore]
 fn bench_block_validation_100m_eth(b: &mut Bencher) {
     let total_validators: usize = 100_000_000 / 32;
     let cycle_length: u8 = 64;

lighthouse/state/block/validation/mod.rs

@@ -1,7 +1,7 @@
 mod validate_ssz_block;
 #[cfg(test)]
 mod tests;
-#[cfg(test)]
+#[cfg(all(feature = "benches", test))]
 mod benches;
 
 use super::attestation_record;