dillon1000/react

Move memoization to begin phase

Currently we update the memoized inputs (props, state) during the
complete phase, as we go back up the tree. That means we can't reuse
work until of its children have completed.

By moving memoization to the begin phase, we can do a shallow bailout,
reusing a unit of work even if there's still work to do in its children.

Memoization now happens whenever a fiber's `child` property is updated;
typically, right after reconciling. It's also updated when
`shouldComponentUpdate` returns false, because that indicates that the
given state and props are equal to the memoized state and props.