Most of the commit phase uses iterative loops to traverse the tree.
Originally we thought this would be faster than using recursion, but
a while back @trueadm did some performance testing and found that the
loop was slower because we assign to the `return` pointer before
entering a subtree (which we have to do because the `return` pointer
is not always consistent; it could point to one of two fibers).
The other motivation is so we can take advantage of the JS stack to
track contextual information, like the nearest host parent.
We already use recursion in a few places; this changes the mutation
phase to use it, too.
Andrew Clark
481dece580