Rename merge state to "bits" and put it into the decision info

Rename merge state to "bits" and put it into the decision info

This makes the returned decision with extra data consist of:
(decision, reason, info, bits, type, from, to, dep)

Merging and printing needs the bits anyway, so providing it
right away makes things easier.

Also add a new SOLVER_DECISIONLIST_MERGEDINFO flag with returnes
"merged" decisions. This is just the "bits" part, the rest
of the decision data stays unchanged.

A new solver_decisionlist_solvables() helper can be used
to return all the decision solvables of a merged or unmerged decision.