Introduce foldl1/2.

This function is exactly lists:foldl/2 save that it cannot be run on the empty
list. The first element of the provided list is the first accumulator.

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>
Merge pull request #9 from blt/foldl1

Introduce foldl1/2.