SET-FUNCALLABLE-INSTANCE-FUNCTION is user interface

It should therefore not trigger AVER failures if the user gets the
order of arguments wrong, but report the problem sanely.  The fix
in this commit is only semi-sane; it would be nice to report an
expected-type of SB-MOP:FUNCALLABLE-STANDARD-OBJECT rather than
SB-KERNEL:FUNCALLABLE-INSTANCE, but there are slightly tricky
bootstrap issues to sort out to get that to work.