Trying to remove as many conditionals as possible from the hottest
codepath: CollectChains. By making SentenceConstruct have modesetting
we eliminate the methods AddTo and GetLatestChain. In the process of
moving PrepareSearch out of CollectChains (and thus out of the Reader
loop in Builder) I found it clearer to group all methods pertaining to
setting up the necessary search vectors, together with the commands,
in a new class: BuilderVectors.