bring back [diskIs], partially reverting commit 933c51fd
bring back [diskIs], partially reverting commit 933c51fd

turns out this might be useful after all, for the buffer cache.
making the whole disk into a list doesn't seem super appealing.

perhaps the mistake was [diskptsto], and not [diskIs] itself.

mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl