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.