Next step 5. capture unroll coin state number at the appropriate time rather than hackily subtracting 1

[prozacchiwawa]

There was a potentially unsafe subtraction that also was doing something sneaky. It was emulating the state number of channel handler before a mutating += 1 everywhere unroll coin's update was called. This captures it at the right time in each case so it doesn't need to subtract 1 when generating the curried unroll coin arguments and should be more durable. Ultimately it'd be nice to remove mutability from channel_handler, but I'm just getting a hold of that in referee, so it'll likely be much later.