Split the balancing algorithm in two parts

Currently the computation, recursing part and the IO part (progress
updates) of the balancing main function (iterateDepth) are all in the
same function, which makes it hard to test. This patch moves the
decision/computation part (whether to proceed one more round, whether we
got a good result, etc.) into Cluster.hs, and leaves only the iteration
and screen update in hbal.hs.