#17 standard concurrency mechanism

[Aaron Hamid]

Possibly refactor various ad-hoc concurrencies to use a
standard concurrency mechanism like
java.util.concurrent, or possibly the 1.4 backport
thereof. For example, Cross object can be made more or
less immutable, and accessor usage, or explicit run()
invocation can queue a stateless CrossAlgorithm
execution in a thread pool.