Non-parallel version of GC

[Edward Z. Yang]

Hello all,

How would people feel about an alternate GC implementation in GHC which
is not parallel?  A GC like this would be simpler to understand, maybe a
little faster when parallel collection is not being used, and (most
importantly for my case) easier to extend with interesting features.

In particular, this implementation would not have gen_workspace or
gc_thread; these would either be unnecessary or folded directly into the
actual generation object.

Thanks,
Edward