Optimize the pickler phase

 - move "erroneous type" diagnostic into a crash recovery handler,
    rather than running it proactively
 - move the "unexpanded macros" check into refchecks.

 Cuts this phase in half, from about 1% of compile time to 0.5%.
Microoptimization in GenBCode

Save a hash lookup on a usually-empty map.
Inline SpecializeGroup.Primitives into Function0

Saves a little time in the specialization InfoTransformer
every time a Function0 subtype is referenced.