Non-parallel version of GC

[Edward Z. Yang]

> But perhaps there's another way to achieve your goals - what are the 
> interesting features you want to add?

Yeah. The key thing I need to change is how the GC decides where
live objects are evacuated to, to support a more efficient implementation
of resource limits (think BiBoP for cost centers) where every user
gets his own set of pages, and his objects are always evacuated to
pages he owns.  I don't know how to parallelize his, and even in
the non-parallel case it requires quite a restructuring of the GC code.

Edward