`Backtrex.solve/1` should return a Stream of solutions

[jmitchell]

Problems can have multiple solutions, but Backtrex currently returns the first one it finds. Instead it should return a lazy Stream of all of solutions (even if there's infinitely many!). It should also provide a function to get the first n solutions and the Stream of the remaining solutions.

Conceptually all this should require is upon finding a solution...

1. enqueue it and
2. backtrack to the next possible state