dillon1000/react

Minimize abuse of .alternate

This avoids using .alternate as much as possible and isolates it
to the root. For anything that is "work in progress" this happens
to be the same as the alternate field. To avoid an extra "current"
field on work in progress fibers, we can just use the alternate.

The timing for when this is true might be a bit tricky to reason
about so I explicitly pass the current value everywhere from the
top. That way we can always change this in the future to use an
explicit field or we can try to maintain a parallel data
structure that remembers which was the "current" fiber for each
wip child.