Avoid work in GenICode#run when inactive.

scalaPrimitives.init() represented 1% of a small (1s)
compilation run.
Optimize generic signatures utility method `dotCleanup`
Fix buggy refactoring to isMacroApplication
Fix buggy change to Symbol.tpe caching
OPT (?) special case empty set concatenation in specializedtypevars

 - cleaner profiles, but not sure if it helps.
 - the specialized info transformer accounts for 2% of compile
   times and in 99% of the time is a no-op.
   Idea: pickle the fact that a class contains no specialization.