feat: optimize runtime (2x+ faster)

tests/programs/pos_local_slots.fzs

@@ -0,0 +1,30 @@
+// Slot indices follow declaration order, not first-use order.
+uint first = 1;
+uint second = 2;
+second = second + 1;
+first = first + second;
+if (first != 4)
+  exit(1);
+
+// Shadowed locals use distinct slots in distinct environments.
+uint value = 10;
+{
+  uint value = 20;
+  value = value + 1;
+  if (value != 21)
+    exit(1);
+}
+if (value != 10)
+  exit(1);
+
+// Access an outer local from nested loop/body scopes.
+uint total = 0;
+for (uint i = 0; i < 4; i = i + 1)
+{
+  {
+    uint nested = i;
+    total = total + nested;
+  }
+}
+if (total != 6)
+  exit(1);