bug#17168: 24.3.50; Segfault at mark_object

[Stefan Monnier]

> This scheme works and passes Dmitry's test, but the resulting
> Vpure_reachable vector has over 8,000 items. Most of these items are
> ordinary interned symbols.

What objects are there besides symbols in Vpure_reachable?
If we can reduce Vpure_reachable to only contain symbols, then we can
replace it with a `pinned' bit in the Lisp_Symbol struct and then walk
the list of symbols during mark, marking all those symbols with the
`pinned' bit.


        Stefan