commitment: replace fold heuristic by func/method

[AskAlexSharov]

in fold we have heuristic:

partsCount := bits.OnesCount16(hph.afterMap[row])
switch partsCount {
	case 0: // Everything deleted
	case 1: // Leaf or extension node
	default: // Branch node
}

need replace it by something more readable:

switch partsCount, t := hph.updateType(); t {
	case UpdateTypeEverythingDeleted: 
	case NewExtension:
        case NewBranch: 
	default: 
}

and such enum must have comments - what is it about, why only 3 values, what is "everything", etc...