Set nodes dirty when sizeThatFits changes

Summary:
In YogaKit, nodes are not dirtied when `sizeThatFits` of leaf nodes change. This makes it difficult to reuse views like `UILabel` when their content (text) changes. I wrote a failing test to prove it and wrote a fix by caching the latest value of `sizeThatFits` in `YGNodeBridge` and checking for changes in `YGAttachNodesFromViewHierachy` (renamed to `YGUpdateNodesFromViewHierachy` to make it clear it's not simply reattaching nodes anymore). When a new size is detected, I call `YGNodeMarkDirty` on the leaf node.

**Unfortunately**, this makes another test fail for no logical reason (`testThatViewNotIncludedInFirstLayoutPassAreIncludedInSecond`). Either there's something I don't understand or I've un-advertedly unearthed a bug in the C implementation, but in what case would setting a node dirty make the layout return a different size?
Closes https://github.com/facebook/yoga/pull/308

Reviewed By: emilsjolander

Differential Revision: D4378826

Pulled By: dshahidehpour

fbshipit-source-id: cd0640bc863debb9e434370739416e3e33e57a08

YogaKit/UIView+Yoga.h

@@ -75,4 +75,9 @@
  */
 - (BOOL)yg_isLeaf;
 
+/**
+ Mark that a view's layout needs to be recalculated. Only works for leaf views.
+ */
+- (void)yg_markDirty;
+
 @end