[runtime] Fix bitfield comments.
[sgen-bridge] Use an extra vtable bit to flag opaque objects.
[sgen-bridge] Fix counters. They were not zeroed after each collection.
[sgen-bridge] Implement type based dfs1 filtering.

Some types are statically known to never lead to a bridge. Objects like
string or int[]. By filtering them out very early in dfs1 we save memory by
not inserting them into the hashtable and reduce work done by all further steps.