select up and down the inspector hierarchy

Summary:
This allows you to select the displayed owner hierarchy, and see the styles,
props, and position.

@public

Test Plan:
Open the inspector, select something in the middle of the page. Click the
breadcrumb train in the inspector, and verify that:
- styles are reflected
- margin/padding/box is correct
- the highlight updates to show the selected item

See video as well.

[Video](https://www.latest.facebook.com/pxlcld/mqnl)

Screenshot
{F22518618}

Libraries/Utilities/mapWithSeparator.js

@@ -0,0 +1,19 @@
+/**
+ * Copyright 2004-present Facebook. All Rights Reserved.
+ *
+ * @providesModule mapWithSeparator
+ */
+'use strict';
+
+function mapWithSeparator(array, valueFunction, separatorFunction) {
+  var results = [];
+  for (var i = 0; i < array.length; i++) {
+    results.push(valueFunction(array[i], i, array));
+    if (i !== array.length - 1) {
+      results.push(separatorFunction(i));
+    }
+  }
+  return results;
+}
+
+module.exports = mapWithSeparator;