Add toArray for HTMLCollection (#109)

* Add `toArray` for HTMLCollection

With a lot of help from @garyb, here's the `toArray` for HTMLCollection.
It returns the elements within the collection as an Array of Element
objects. It was suggested that this be returned as an Unfoldable, but
the `item` function is effectful, so the plan was ruined.

* Update toArray to use FFI

It's not great, but it's going to be more performant and just as
"correct" (for some definition of correctness).

Author: Tom Harding

src/DOM/Node/HTMLCollection.js

@@ -6,6 +6,12 @@ exports.length = function (list) {
   };
 };
 
+exports.toArray = function (list) {
+  return function () {
+    return [].slice.call(list);
+  };
+};
+
 exports._item = function (index) {
   return function (list) {
     return function () {

src/DOM/Node/HTMLCollection.purs

@@ -14,6 +14,9 @@ import DOM.Node.Types (Element, HTMLCollection, ElementId)
 -- | The number of elements in a HTMLCollection.
 foreign import length :: forall eff. HTMLCollection -> Eff (dom :: DOM | eff) Int
 
+-- | The elements of an HTMLCollection represented in an array.
+foreign import toArray :: forall eff. HTMLCollection -> Eff (dom :: DOM | eff) (Array Element)
+
 -- | The element in a HTMLCollection at the specified index, or Nothing if no such
 -- | element exists.
 item :: forall eff. Int -> HTMLCollection -> Eff (dom :: DOM | eff) (Maybe Element)