Replace AssertionTree with predicate annotations

[robdockins]

The AssertionTree datastructure does not have explicit sharing, and is probably the reason for the slowdown described in #407.

Rather than rework this datastructure to preserve sharing, I think it makes more sense to allow what4 predicates to be labeled with annotations. These annotations can later be traversed to build the kind of user-visible explanations that AssertionTree is currently used for. This will both simplify the API and, hopefully, alleviate a performance problem.

[robdockins]

CF #441.

[robdockins]

Annotations are now used instead of assertion trees. Next we need to implement #462 to actually use the embedeed annotations.