Representation theorem re-uses dummy names.

Before, the n0 name in the representation theorem was always a
completely new name. As Grounder::PrepareForQuery() was called for the
formula containing n0, the name was considered for grounding from then
on, even though no other formula would ever mention n0.

Now there are Grounder::CreateName() and Grounder::ReturnName() to
temporarily use names. KnowledgeBase::ResOtherName() uses them.

The performance impact may be huge when many queries with
quantifying-in are posed. For example, test-introspection.txt is about
100 times as fast now.