Add a compacting garbage collector.

It isn't enabled by default, as there are still a couple of problems:
there's a fallback case I haven't implemented yet which means it will
occasionally bomb out, and speed-wise it's quite a bit slower than the
copying collector (about 1.8x slower).

Until I can make it go faster, it'll only be useful when you're
actually running low on real memory.

'+RTS -c' to enable it.

Oh, and I cleaned up a few things in the RTS while I was there, and
fixed one or two possibly real bugs in the existing GC.