Prevent inappropriate byes being awarded to submissions during dispute

[area]

Byes were able to be awarded too liberally in the current codebase. At the very least, byes should never have been able to be claimed if the submission window was still open (because more submissions might be made). In addition, byes should never be able to be awarded if there are any submissions from any previous rounds still not eliminated.

I took several cracks at this before settling on this solution, but I'm still not happy with it. The tricky bit is preventing the award of a bye if there are any submissions from previous rounds not eliminated yet. We can't directly track if a round is complete with a flag, because multiple rounds may silently complete if the window closes and all existing disputes have completed.

A loop through all previous rounds is the natural way of solving this, but loops are bad. I've allowed myself to be a little clever and short-circuit it a bit so that if there are a series of disputes where multiple byes should be awarded (as is the case if there are e.g. 11 submissions and none time out). In that scenario, only the previous rounds that weren't complete before are examined. While the unbounded loop should be fine with our current configuration (because the maximum number of rounds should be capped at 14, based on the block gas limit and the current timeout), I've added this extra element in in an attempt to harden the approach if the parameters change in the future.

Also fixed a bug in accommodateChallenge... with the return from mod(2) - at some point, it was a JS number and is now not? This was noted as a breaking change for v5 of bn.js but I can't figure out how (if?) we were exposed to that...