dillon1000/react

Add Fragment fiber type

We currently treat nested arrays as a unique namespace from top
level children. So that these won't share key namespaces. This
adds a new fiber type that will represent the position of a
fragment.

This is only used for nested arrays. Even if you return an array
from a composite component, we don't need this since they share
namespace with a single top level component.

This still doesn't implement the complete reconciliation
algorthim in child fiber. That's coming later.