ambit 0.9.0

This is an all-ruby implementation of choose/fail nondeterministic programming with branch cut, as described in Chapter 22 of Paul Graham's <em>On Lisp</em>[1], Chapter, or Section 4.3 of <em>SICP</em>[2]. Due to Ruby containing a true call/cc, this is a much straighter port of Paul Graham's scheme version of this code than his Common Lisp or my C versions are. :-)