Move internal modules to “internal” folder.

.internal/memoizeCapped.js

@@ -0,0 +1,26 @@
+import memoize from './memoize.js';
+
+/** Used as the maximum memoize cache size. */
+const MAX_MEMOIZE_SIZE = 500;
+
+/**
+ * A specialized version of `memoize` which clears the memoized function's
+ * cache when it exceeds `MAX_MEMOIZE_SIZE`.
+ *
+ * @private
+ * @param {Function} func The function to have its output memoized.
+ * @returns {Function} Returns the new memoized function.
+ */
+function memoizeCapped(func) {
+  const result = memoize(func, key => {
+    if (cache.size === MAX_MEMOIZE_SIZE) {
+      cache.clear();
+    }
+    return key;
+  });
+
+  const cache = result.cache;
+  return result;
+}
+
+export default memoizeCapped;