Hide benchmarks behind a feature flag (#5024)

* Hide benchmarks behind a feature flag

* Propage attributes in impl_runtime_apis macro

* Bump impl_version

* Fillter cfg attributes

* Hide more things under the feature

* Fix set_block_number availability

* Rename filter_attrs -> filter_cfg_attrs

* Rename runtime_benchmarks to runtime-benchmarks

substrate/frame/system/Cargo.toml

@@ -36,6 +36,7 @@ std = [
 	"sp-runtime/std",
 	"sp-version/std",
 ]
+runtime-benchmarks = []
 
 [[bench]]
 name = "bench"

substrate/frame/system/src/lib.rs

@@ -885,7 +885,7 @@ impl<T: Trait> Module<T> {
 
 	/// Set the block number to something in particular. Can be used as an alternative to
 	/// `initialize` for tests that don't need to bother with the other environment entries.
-	#[cfg(any(feature = "std", test))]
+	#[cfg(any(feature = "std", feature = "runtime-benchmarks", test))]
 	pub fn set_block_number(n: T::BlockNumber) {
 		<Number<T>>::put(n);
 	}