Track deletions using an array on the parent

Adds back the `deletions` array and uses it in the commit phase.

We use a trick where the first time we hit a deletion effect, we commit
all the deletion effects that belong to that parent. This is an
incremental step away from using the effect list and toward a DFS +
subtreeFlags traversal.

This will help determine whether the regression is caused by, say,
pushing the same fiber into the deletions array multiple times.