Pre GC sanity checking

[qinsoon]

Our current sanity checking happens after the main GC. This becomes less useful for binding developers if they fail in the GC and the sanity checking has no chance to run.

It will be helpful if we can do sanity checking before a GC. The best timing to do so would be after Prepare and before Closure. We need to confirm if we have the complete root set at this point.

[k-sareen]

I thought Angus had already implemented something very similar? We should try and upstream it if possible.