[sgen] Refactor 'alloc_handle()'.
[sgen] Remove magic numbers.
[sgen] Avoid redundant calls in 'mono_gchandle_iterate()'.
[sgen] Update binary protocol for dislinks.
[sgen] Fix slot hint in GC handle allocation.
[sgen] Skip unused slots when iterating GC handles.
[sgen] Properly reveal hidden pointer.
[sgen] Remove pointer hiding for GC handles.