refactor(immut): if-is for None=>() match in HAMT each helpers

Both `immut/hashset/HAMT.mbt` and `immut/hashmap/HAMT.mbt` had a noop-on-empty
match in their iteration helper:

```moonbit
match self.0 {
  None => ()
  Some(node) => go(node)
}
```

Replaced with the equivalent direct form:

```moonbit
if self.0 is Some(node) {
  go(node)
}
```

Both files share the exact same shape so bundled.

Semantics-preserving: full `moon test` (6425/6425) passes and `moon info`
produces no `.mbti` change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: bobzhang

immut/hashmap/HAMT.mbt

@@ -610,9 +610,8 @@ pub fn[K, V] HashMap::each(
     }
   }
 
-  match self.0 {
-    None => ()
-    Some(node) => go(node)
+  if self.0 is Some(node) {
+    go(node)
   }
 }
 

immut/hashset/HAMT.mbt

@@ -341,9 +341,8 @@ pub fn[A] HashSet::each(
     }
   }
 
-  match self.0 {
-    None => ()
-    Some(node) => go(node)
+  if self.0 is Some(node) {
+    go(node)
   }
 }