bug#36649: 27.0.50; pure space and pdumper

[Paul Eggert]

> I think we have the following options:
>
> 1. remove pure space entirely
> 2. remove pure space, but leave PURE_P and CHECK_IMPURE as reminders
> to do something about it.
> 3. move pure space to BSS
> 4. xmalloc() pure space, only when needed
> 5. modify pdumper to mark and recognize pure objects
> 6. do nothing and accept the wastefulness

I suggest (1), since it will result in simpler code. Although (2) would be OK 
too, if we ever introduce immutable objects it's likely that PURE_P and/or 
CHECK_IMPURE will just get in the way anyway. We can see a hint of that in my 
patch today that added temporary immutability to hash tables to fix a core-dump bug.