File-based response support & static server

- Clean up UniversalPath implementation
    - Use Readable/Writable types correctly for stream methods
    - Add .list() methods for getting child files

- Make Response body specify explicit types and support
  writing Readable streams to the body

- Create a static file server that supports directory listing

src/util/collection/Collection.ts

@@ -50,6 +50,20 @@ class Collection<T> {
         return new Collection(items)
     }
 
+    /**
+     * Create a new collection from an item or array of items.
+     * Filters out undefined items.
+     * @param itemOrItems
+     */
+    public static normalize<T2>(itemOrItems: (CollectionItem<T2> | undefined)[] | CollectionItem<T2> | undefined): Collection<T2> {
+        if ( !Array.isArray(itemOrItems) ) {
+            itemOrItems = [itemOrItems]
+        }
+
+        const items = itemOrItems.filter(x => typeof x !== 'undefined') as CollectionItem<T2>[]
+        return new Collection<T2>(items)
+    }
+
     /**
      * Create a collection of "undefined" elements of a given size.
      * @param size