Move internal modules to “internal” folder.

.internal/setToPairs.js

@@ -0,0 +1,18 @@
+/**
+ * Converts `set` to its value-value pairs.
+ *
+ * @private
+ * @param {Object} set The set to convert.
+ * @returns {Array} Returns the value-value pairs.
+ */
+function setToPairs(set) {
+  let index = -1;
+  const result = Array(set.size);
+
+  set.forEach(value =>
+    result[++index] = [value, value]
+  );
+  return result;
+}
+
+export default setToPairs;