Add back deprecated getParent methods for non-breaking API change (#775)

Summary:
I'm not totally sure what I'm doing so if this needs changes let me know.
Closes https://github.com/facebook/yoga/pull/775

Reviewed By: emilsjolander

Differential Revision: D8331892

Pulled By: passy

fbshipit-source-id: eb1023e666322d2472e4081fd4a4e72a7b43d049