Introduce tfoldl/3.

This function allows you to foldl over a tree structure. This resolves #26.

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>
Merge pull request #31 from blt/foldl

Introduce tfoldl/3.
Realized a simpler way of implementing tfoldl.

In effect, previously, the internal functions were re-implementing a clause of
top-level tfoldl. This commit corrects that.

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>
Merge pull request #32 from blt/foldl

Realized a simpler way of implementing tfoldl.