Improve Call Arity performance

This improves how the Call Arity deals with "boring" variables. Boring
variables are those where it does not bother to include in the analysis
result, so whenever something is looked up in the analysis result, we
have to make a conservative assumption about them.

Previously, we extended the result with such conservative information
about them, to keep the code uniform, but that could blow up the amount
of data passed around, even if only temporarily, and slowed things down.

We now pass around an explicit list (well, set) of variable that are
boring and take that into account whenever we use the result. Not as
pretty, but noticably faster.