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/link.rs

@@ -135,7 +135,10 @@ pub fn link<A>(target: Pid<A>) {
 
     if registered_on_target {
         with_runtime(|inner| {
-            let slot = inner.slot_at(me).expect("link: own slot vanished");
+            let slot = match inner.slot_at(me) {
+                Some(s) => s,
+                None => panic!("smarm: link own slot vanished (core corrupt)"),
+            };
             let mut cold = slot.cold.lock();
             if !cold.links.contains(&target) {
                 cold.links.push(target);