Move internal modules to “internal” folder.

.internal/isKeyable.js

@@ -0,0 +1,15 @@
+/**
+ * Checks if `value` is suitable for use as unique object key.
+ *
+ * @private
+ * @param {*} value The value to check.
+ * @returns {boolean} Returns `true` if `value` is suitable, else `false`.
+ */
+function isKeyable(value) {
+  const type = typeof value;
+  return (type == 'string' || type == 'number' || type == 'symbol' || type == 'boolean')
+    ? (value !== '__proto__')
+    : (value === null);
+}
+
+export default isKeyable;