Speed up median

Drive-by removal of unwrap in use of `Arc` in `NumbatList::head`

numbat/modules/math/statistics.nbt

@@ -45,7 +45,8 @@ fn stdev<D: Dim>(xs: List<D>) -> D = sqrt(variance(xs))
 @example("median([1 m, 2 m, 400 cm])")
 fn median<D: Dim>(xs: List<D>) -> D =  # TODO: this is extremely inefficient
   if mod(n, 2) == 1
-    then element_at((n - 1) / 2, sort(xs))
-    else mean([element_at(n / 2 - 1, sort(xs)), element_at(n / 2, sort(xs))])
+    then element_at((n - 1) / 2, sorted)
+    else (element_at(n / 2 - 1, sorted) + element_at(n / 2, sorted)) / 2
   where
     n = len(xs)
+    and sorted = sort(xs)

numbat/src/list.rs

@@ -113,12 +113,9 @@ impl<T: Clone> NumbatList<T> {
     /// clone the value that's being returned.
     pub fn head(self) -> Option<T> {
         let front = self.view.map_or(0, |(start, _end)| start);
-        if Arc::strong_count(&self.alloc) == 1 {
-            // safety: unwrap cannot fail because we ensured there was only one strong reference above
-            let mut alloc = Arc::try_unwrap(self.alloc).map_err(|_| ()).unwrap();
-            alloc.swap_remove_front(front)
-        } else {
-            self.alloc.get(front).cloned()
+        match Arc::try_unwrap(self.alloc) {
+            Ok(mut solely_owned) => solely_owned.swap_remove_front(front),
+            Err(shared) => shared.get(front).cloned(),
         }
     }