src/class-make-impl.lisp: Don't store `nil' in the `metaclass' slot.

Normally we can fill `metaclass' in at construction time, but this is
difficult while we're bootstrapping the class graph.  Previously, we'd
store `nil' in the slot, and expect `bootstrap-classes' to fix things up
later; but actually, the `metaclass' slot is declared to hold only
`sod-class' objects.

Rather than expand the slot type, delay the `guess-metaclass' machinery
until class finalization (moving the code across into the relevant
source files).