Use a variety of A* algorithm to speed up calculations

The [A* search algorithm](https://en.wikipedia.org/wiki/A*_search_algorithm) searches from both the start and end points to find a path which meets in the middle. This is significantly quicker than a simple breadth- or depth-first algorithm.

I believe we could us a similar technique to find solutions to the numbers round faster, by manipulating the target number (such as dividing it by an available solution number) and then seeing if we can reach the new target without using the numbers we used to get there.

I'm not explaining this very well - it's like we're doing two equations and trying to get them to meet in the middle without using numbers more than once.

The issue names no files, tests, or entry points. First locate the current Countdown numbers solver and clarify how a bidirectional search would preserve the rule that numbers are not reused; done means valid solutions are still found with faster calculations.

Written by the indexing model from the issue text.