dillon1000/react

[Fiber] Remove output field (#8406)

* Remove output field

The idea was originally that each fiber has a return value. In practice
most of what we're modelling here are void functions and we track side
effects instead of return values.

We do use this for coroutines to short cut access to terminal yields.
However, since this can be nested fragments we end up searching the tree
anyway. We also have to manage this in transferOutput so it ends up being
as expensive. Maybe we can save some traversal for updates when coroutine
branches bail out but meh.

* Unmount child from the first phase of a coroutine