Extract qualifs per local from ConstCx

To do dataflow-based qualification, we need to pass a reference to the
dataflow state directly to the `Qualif::in_*` methods. As such, we
move the `PerQualif<BitSet<Local>>` that holds the qualification state
for each local to `Checker` itself and use a macro (`get_qualif`) to
extract the correct `BitSet` when querying the qualifications for each
local.

Author: ecstatic-morse