Collect more GC stats (for Julia)

[qinsoon]

See for details mmtk/mmtk-julia#46.

• Allocation volume
• Max GC pause
• Time to safepoint
• Max time to safepoint
• Sweep time (time in Release?)
• Mark time (time in transitive closure?)
• Moved object ratio

It is not necessary to use Stats. As long as we can collect the data, and get the data into the binding, that would be enough. But if the metric is also useful to other bindings and our performance analysis, we can use Stats and print into our statistics output.