Moved the creation of accessor generic functions to later.

Creating an accessor generic functions requires the creation of an
initial discriminating function, so we first need to make sure that
the machinery for creating discriminating functions is in place.
Call compute-discriminating function to create initial one.
Skeleton version of COMPILE in phase 3.

Right now, this file only contains proof of concept.  It redefines
COMPILE to call ALLOCATE-INSTANCE of the built-in class FUNCTION.  The
first improvement will be to call MAKE-INSTANCE instead so that the
instance gets properly initialized.  Once the cross compiler works, it
will call the cross compiler to generate a target function.
Include new version of COMPILE as a component in phase 3.