Added a simple but general backtracking generator (conjoin), and a couple

Added a simple but general backtracking generator (conjoin), and a couple
examples of use.  These poke stuff not specifically targeted before, incl.
recursive local generators relying on nested scopes, ditto but also
inside class methods and rebinding instance vars, and anonymous
partially-evaluated generators (the N-Queens solver creates a different
column-generator for each row -- AFAIK this is my invention, and it's
really pretty <wink>).  No problems, not even a new leak.