Count malloc size in pages

[qinsoon]

We should count malloc size by pages in malloc mark sweep. And when we provide counted malloc, we should also count in pages (which means we will need to maintain some metadata to track which page is used in MMTk for those malloc results).

We probably will share some code between those two. #572 was trying to turn the current malloc marksweep space to a manual malloc space. We may not directly use code from the pull request.