Liberalise the defaulting rules for GHCi

		Merge to STABLE?

The H98 defaulting rules are these.  Group constraints of the form (C v),
for some particular type variable v. Then default v if

a) v appears only in this group (no implicit params or D [v])
b) at least one of the C's is numeric
c) all the C's are standard

GHCi changed rules (b) to

(b') at least one of the C's is numeric, or Eq,Ord,Show

This commit further liberalises GHCi to change (c):

(c') any of the C's are standard

Otherwise the existence of a non-standard class (e.g. Random) is enough
to kill defaulting altogether.