There were a couple of problems with the implementation:

 - the pre hook was called before yielding a directory. This isn't
   wrong so much as inconvenient -- you can't indent on pre-, and
   outdent on post-, for example.

 - the post hook was called at the top of the stack, i.e., when
   leaving the starting point, so everything had some variety of
   off-by-one problem.

This commits makes the tests a bit more rigorous (the post-hook test
in particular didn't check that the number of times leaving a
directory was equal to the number of times entering a
directory!). I've also given the walk procedure some typedocs, so it
has a better explanation in the API reference.