[sgen] Binary protocol entries for scanning objects and vtypes.
[sgen] Fix build of sgen-grep-binprot.
[sgen] Fix names of descriptor types in describe_ptr().
[sgen] Cementing bugfix for concurrent M&S.

We have to pin cemented objects at the finishing collection pause, too.  If we don't,
we don't scan them, and if they reference major objects that aren't referenced
elsewhere, we don't mark those major objects, and we break.