Combine sweeping and moving

This commit combines the sweep step with moving objects.  With this
commit, we can do:

```ruby
GC.start(compact: true)
```

This code will do the following 3 steps:

1. Fully mark the heap
2. Sweep + Move objects
3. Update references

By default, this will compact in order that heap pages are allocated.
In other words, objects will be packed towards older heap pages (as
opposed to heap pages with more pinned objects like `GC.compact` does).

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