Fix compilation error `use of unstable library feature 'stdsimd'`

• Upgraded to latest nightly to closes #193 (closed)
• Fixed clippy warnings.
• Fixed one compilation error with try-runtime feature and the unused import and dead code warnings. G1Executor and GTest are only utilized with runtime-benchmarks, whereas GDevExecutor is used with either runtime-benchmarks or try-runtime. Rather than complicating the code with error-prone (or impossible) feature combinations, I've opted to allow unused import and dead code within this particular section.

node/src/command.rs

@@ -15,6 +15,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#![allow(unused_imports)]
+
 pub mod key;
 pub mod utils;
 
@@ -24,10 +26,8 @@ use crate::cli::{Cli, Subcommand};
 #[cfg(feature = "g1")]
 use crate::service::g1_executor::G1Executor;
 #[cfg(feature = "gdev")]
-#[cfg(feature = "runtime-benchmarks")]
 use crate::service::gdev_executor::GDevExecutor;
 #[cfg(feature = "gtest")]
-#[cfg(feature = "runtime-benchmarks")]
 use crate::service::gtest_executor::GTestExecutor;
 use crate::service::{IdentifyRuntimeType, RuntimeType};
 use crate::{chain_spec, service};