JSON marshalling for save/restore of Hll in progress

The idea is that there are two modes:
 - sparse mode: isSparse==true, sparse!=nil, bigM==nil
 - dense mode: isSparse==false, sparse==nil, bigM!=nil

Each Hll starts in sparse mode and switches to dense mode once it reaches a certain size.

It was a pretty serious bug that we were allocating memory for bigM in sparse mode.