move to 0.27
sort writables to be able to skew write probability to lesser ids (older volumes)
add error return for (Un)GzipData
add cmd/dump - a dumper

Walk needed to be added to NeedleMap and CompactMap, to be able to add WalkKeys and WalkValues to volume. This is needed for iterating through all the stored needles in a volume - this was dump's purpose.