Use symmetry

[porglezomp]

We're working with a 5x5 grid, so we've got lots of symmetry we could take advantage of:

• Horizontal/vertical reflections
• 90º rotations

Depending on how the patterns collide, this could cut down the total size of the cache by 2x, 4x, 8x even.

Implementation ideas:
Have some system for deciding which orientation of a board is canonical (the numerically lowest one?) and only store that one, transforming all boards into canonical form before they're handled.