Generalization of Sanity GC

[qinsoon]

Currently sanity GCs happen before and after a normal GC, and are used to check if the normal GC is correct.
We would like to have a general sanity GC/analysis that can be used in two places:

1. before and after a normal GC, as how it is functioning now.
2. standalone. We should be able to run a sanity analysis without collecting anything. We would still need to get roots from mutator, stop the world, etc. However, it has no side effect, and does not do reclamation.

[qinsoon]

This is done with #198, #227 and #248.