get SPECIALIZER-DIRECT-METHODS (and -GENERIC-FUNCTIONS) right

get SPECIALIZER-DIRECT-METHODS (and -GENERIC-FUNCTIONS) right

we can do what we need by having SB-MOP:ADD-DIRECT-METHODS and
SB-MOP:REMOVE-DIRECT-METHODS maintain a per-class table, indexed by
the specializers under SB-PCL::SAME-SPECIALIZER-P, of direct methods.
This then is agnostic to whether the specializers are interned or
freshly generated.