dillon1000/react

Added ReactFiberStack shared by ReactFiberContext and ReactFiberHostContext ReactFiberStack is the new underlying stack used by ReactFiberContext and ReactFiberHostContext. The goal is to simplify the 2 context managers and to add more errors/dev-warnings when we pop unexpected. This changeset currently causes a lot of tests to fail (as we are currently popping too many times and/or in the wrong order). I'm pushing this branch now to share with Sebastian as he is working on a related cleanup pass at beginWork.