Refactor marksweep policy

[qinsoon]

Currently we have a MallocSpace policy which uses the library malloc as the freelist allocator and does mark/sweep. As we will have our own free list allocator (#348), we should refactor the existing MallocSpace to extract the common mark sweep code for a MarkSweepSpace while separating the code for the global part of the free list allocator.

What we have now for the marksweep plan is MallocAllocator + MallocSpace.
For this refactoring, we should have: MallocAllocator + MarkSweepSpace<MallocAllocator>. The marksweep space should replace the current MallocSpace with a clear interface between the global freelist allocator and the policy. We may later have another plan MiMallocAllocator + MarkSweepSpace<MiMallocAllocator>.

We need to be careful about the freelist allocator. For a library malloc, the address may be outside of our normal heap range, while for our 'native' freelist allocator, the address is always in our heap range. For sweeping, a library allocator can only do eager sweeping. But for our 'native' allocator, we should support both eager and lazy sweeping.

[k-sareen]

I think the interface should be something like the expected default allocator interface (i.e. alloc(), etc.) + checking if an object is marked and if it has been alloc'd. Other than that, we have to be careful regarding the malloc library specific chunk and page accounting metadata and code.