Revert "Schema{Walker,Listener}: change API"

This reverts commit 84790ceef4ad4ed64a6c071a65ec2f9497e9be9e.

Bad idea. It may be that we actually want to visit some parts of the tree.

This means pointer collectors will need to be smarter than they currently are.
Also, this smoothes the transition for syntax checkers, which will eventually
forget about collecting subpointers.