[sgen] Optimized mark/scan loop for major collections.

[sgen] Optimized mark/scan loop for major collections.

The basic idea here is to make a loop that combines dequeuing from the
gray queue, scanning and marking/copying/enqueuing, with redundancies
removed and special-casing of the common case.

We make the small bitmap type descriptor only apply to small (as
opposed to LOS) objects so that we can quickly check in that case
whether an object is in the LOS or not without actually looking up its
size.

We disable evacuation and the split nursery because the code doesn't
support them yet.