Documented Queens and added (and tested) some more basic generators.
Documented the advanced nqueens generators.
Made Tree a submodule of Utils rather than Examples.

This was done because Tree as it currently exists
does not provide a useful example of how to build
a search tree using TreeGenerator, but rather was
intended to factor out common code for searching
simple trees.
Documented the Tree module.
Inserting 10 microsecond delays *may* have fixed the segfaults.
Shortened the names of the checkpoint constructors.
merge*Checkpoint* --> simplify*Checkpoint*
Tweaked some of the docs wording in the Vistior module.
Documented Checkpoint, in the process removing the walk functions.

When I had written them they seemed like they were
going to be more useful than they actually turned
out to be, and more frustratingly as I add new
visitor modes it takes a non-trivial amount of
effort to update these functions by creating a new
fetcher datatype and a new test.

Thus, I've decided to just throw them all out
since it is unlikely that anyone will have ever
cared about them and if they do I can always
resurrect them.
Added a direct test of the steppers in Checkpoint.