Temporarily remove satisfied clauses from watchlists

[Jannis Harder]

This is an idea I have for optimizing unit propagation. It tries to avoid work in the following scenario:

• A clause C = x, y1, ..., yn is satisfied by a single literal x which was assigned on a fairly low decision level
• It's other literals y1, ..., yn get repeatedly assigned false and unassigned again on deeper decision levels

I need to benchmark how often this happens, but I figure it should be common.

The standard algorithm would repeatedly encounter the clause C while scanning the watchlists for y1, ..., yn. Blocking literals (already implemented) make sure that the second time this happens, we don't have to access the clause itself, as the watch will contain x, which is true, as blocking literal.

I think it's possible to further improve this by temporarily moving the watch to a inactive-watches list associated with x. During backtracking when x is unassigned the inactive-watches list is then emptied and the watches are moved back.

I can also imagine this to be especially useful for solving with assumptions, where we essentially never unassign some literals.

This might have synergies with #20.

[Jannis Harder]

While this does happen frequently (seems to be 10% to 50% of watches which have a true blocking literal for the instances I looked at) it doesn't happen often enough in a row to outweigh the overhead of moving the watch somewhere else before moving it back.