Genericize for any usage.
* adapt it to other (non-boardgame) workloads
  * Break the players=2 assumption
    * Get the necessary metadata into the Physics plugins, to allow the
      tournament system to figure out how many critters to put into each match.
    * Handle the n=1 case
    * Handle the n>2 case
  * Break the direct-competition assumption
    * Physics->double_match should become an internal method; it shouldn't be
      called directly.
    * Maybe Physics plugins should implement a specific ->fight()

More Physics plugins!
* Go
* Speech recognition
* Weather prediction

* I know it works on linux
* I doubt it works anywhere else.  (patches and test reports welcome)