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/timer.rs

@@ -214,7 +214,10 @@ impl Timers {
         let mut out = Vec::new();
         while let Some(r) = self.heap.peek() {
             if r.0.deadline <= now {
-                let entry = self.heap.pop().unwrap().0;
+                let entry = match self.heap.pop() {
+                    Some(e) => e.0,
+                    None => panic!("smarm: timer heap pop after peek returned None (core corrupt)"),
+                };
                 if matches!(entry.reason, Reason::Send { .. }) && !self.armed.remove(&entry.seq) {
                     // Cancelled before it came due: discard, do not deliver.
                     continue;