Kernel depends on MorphicExtras (the SqueakPageCache class in
particular) in two places:

    ObjectOut >> comeFullyUpOnReload
    ObjectOut >> xxxFetch

I really don't like something called "Kernel" (with the implication
that it's the innermost part of the system) depending on Morphic, let
alone MorphicExtras (whose name suggests that it's something entirely
optional).

How do we break this dependency? (It looks hairy!)