Clean up translated assertions and simple generated patterns

[Medowhill]

Summary

• Rewrite C2Rust __assert_fail assertion blocks to assert! in the libc pass, and fold re-expanded assert panics back to assert! in unexpand.
• Extend simpl cleanup for generated Some(...).unwrap(), *& patterns, and empty block statements such as {} / 'l: {};.
• Drop the unnecessary structural_match feature during unexpand cleanup.