Note
Make append() as minimal as possible.
Switch back to unsigned for each.  5-6% off now.
Assert length != 0 in append.  -2% now.
Eliminate double branches in merge.
Use setCount/set instead of setReserve/append in merge.
simpler logic in coverageToAlpha
append -> set here too
comments and factoring
start tweaking DenseSuperBlitter.  Currently +5-6%!