core: rewrite panic sites as explicit match+panic

Replace implicit unwrap()/expect() in the lock-ordered core with explicit
match arms. Lock-poison sites use one uniform message
("smarm: <lock> lock poisoned (core corrupt): {e}"); invariant sites panic
with a descriptive message naming the violated invariant. No behaviour
change: each rewrite preserves the prior panic-on-bad-arm semantics. Also
clears the accompanying clippy hygiene in these files (redundant_closure,
len_without_is_empty, too_many_arguments, unnecessary_sort_by,
missing_safety_doc, nonminimal_bool/unnecessary_unwrap).

src/actor.rs

@@ -93,8 +93,10 @@ pub fn take_last_outcome() -> Option<Outcome> {
 /// unwinding to cross the boundary, but `catch_unwind` here means unwinding
 /// never actually does.
 pub extern "C-unwind" fn trampoline() {
-    let b = CURRENT_ACTOR_BOX.with(|c| c.borrow_mut().take())
-        .expect("trampoline entered without a closure set");
+    let b = match CURRENT_ACTOR_BOX.with(|c| c.borrow_mut().take()) {
+        Some(b) => b,
+        None => panic!("smarm: trampoline entered without a closure set (core corrupt)"),
+    };
 
     let outcome = match panic::catch_unwind(panic::AssertUnwindSafe(b)) {
         Ok(())       => Outcome::Exit,