The types get hairier, but the code is much simpler!
I've left {check,apply}DepthM{,2} there for now, but reimplemented them in
terms of the new combinators.
Fixes#58.
This is solely because GHC 6.6 doesn't like them (it complains about the type
variable already being in scope -- which it is, but there's nothing I can do
about that!). This doesn't lose any safety; if you try to write a
transformation for something that's not Data you'll find out when you try to
pass it to one of the application functions.
We now have three kinds of canned tree traversals, all of which are smart about
which types they're applied to: explicit-descent transformations,
implicit-descent transformations, and implicit-descent checks. I've only
provided depth-first application of the latter two, but we could do
breadth-first in the future if necessary.
