Optimization tips guide for Backtrex users

[jmitchell]

While working on #5, #6, and #3 we're likely to find bottlenecks in the examples. As they're discovered, make note of them in an ongoing experience report. Periodically distill these findings into a guide for Backtrex users wanting to optimize their callbacks.

[jmitchell]

A general optimization strategy is to prioritize the order in which unknowns and values are returned by the callbacks. For example, in Sudoku ordering the blank cells randomly would generally perform worse than prioritizing cells with the most constraints imposed by its row, column, and sector. Similarly, prioritizing the least common values in the current cell's row, column, and sector (or even the entire grid) is likely better than the current 1..9 approach.

[jmitchell]

Finish #12 (Generate profiling reports) first.