Records what the prototype pass is actually for, since it is the first
thing anyone hits: for a single module it is redundant - the generator
infers return types from the bodies it can see, and the prototypes only
save a defensive As* conversion - but for anything multi-module it is
required, and a function returning several values is the case that
cannot be guessed at all.
Also adds the two things the new arithmetic probe turned up to the list
of mistakes the probes caught, and records DISPLAY of an unassigned
expression as a known gap rather than leaving it implied by what the
probe does not test.
