PCH assumes there is no information before the object

[Daniel Berlin <dberlin at dberlin dot org>]

I'm implementing PCH for the new zone collector in my spare
time, and currently, PCH assumes it can simply read in the object at the correct
address, and that will put everything right.
This may work for ggc-page, but it won't work for ggc-simple (which has
no PCH implemented), or the new zone collector.  This is because both put
information before ggc allocated objects.

There seems to be no way to support this without significantly modifying
the PCH reading/writing machinery, which assumes it only has to write the
object out.
I can't even just make a table of the object header info, and write it
out in the zone collector, because we'd still need to put it before each
object, which we have no hook for.

Geoff, have i missed something?

If not, would you prefer i add a new ggc hook called before
writing/reading each object that we can use to write/read the header info,
or do this some other way?