Perf: minor optimization here and there

Build/lib/set-add-from-array.ts

@@ -0,0 +1,8 @@
+/**
+ * In-place adding of elements from an array to a set.
+ */
+export function setAddFromArray<T>(set: Set<T>, arr: T[]): void {
+  for (let i = 0, len = arr.length; i < len; i++) {
+    set.add(arr[i]);
+  }
+}