Added a non-recursive implementation of conjoin(), and a Knight's Tour

Added a non-recursive implementation of conjoin(), and a Knight's Tour
solver.  In conjunction, they easily found a tour of a 200x200 board:
that's 200**2 == 40,000 levels of backtracking.  Explicitly resumable
generators allow that to be coded as easily as a recursive solver (easier,
actually, because different levels can use level-customized algorithms
without pain), but without blowing the stack.  Indeed, I've never written
an exhaustive Tour solver in any language before that can handle boards so
large ("exhaustive" == guaranteed to find a solution if one exists, as
opposed to probabilistic heuristic approaches; of course, the age of the
universe may be a blip in the time needed!).