[sgen-bridge] Don't merge xrefs in DFS2. Speeds up some cases by orders of magnitude.
[sgen-bridge] New xrefs graph optimization for nodes with incoming order 1.
[sgen-bridge] Make merging in old xrefs algorithm much more efficient.

Instead of merging the two lists by keeping them sorted, use the flag on
the SCCs the indicate whether an SCC has already been merged in, and
just append it if not.  This makes it as fast as the new algorithm on one
test case where it was pathologically slow, while not exhibiting another
pathology that the new algorithm has.
[sgen] Heavy hash table statistics.
[sgen-bridge] Remove dead DynArray merging code.
[sgen-bridge] Copy-on-write for DynArrays.

This eliminates a pathological case for the old algorithm, where a
large array of SCCs is copied from one node to the next without
modification.
[sgen-bridge] Added test case with bridge performance pathologies.

This is not meant as part of the test suite, but as a reference for
where the bridge has performance issues.
[sgen-bridge] Remove single-reference non-bridge objects from graph.
[sgen-bridge] Debugging option to dump whole bridge object graph.
[sgen-bridge] object_is_live() should be called object_needs_expansion().
[sgen-bridge] Add a new bridge (non-)pathology.
[sgen-bridge] Separate DFS1 and DFS2 fields in HashEntry.
[sgen-bridge] Remove redundant object pointer in hash table entry.
[sgen-bridge] Fix forwarding.
We can't forward to HashEntry's because they are volatile - the
hash table will resize, and the entries move.  We need to point to
the objects, and then look them up via the hash.  Also, we have
to forward chains of forwards.
[sgen-bridge] Make finishing_time a 32 bit unsigned int.
[sgen-bridge] Use bit field for is_visited and finishing_time.
[sgen-bridge] Special-case ptr array for 1 entry without allocating.
[sgen-bridge] Separate before- and after-callback in new bridge.
[sgen-bridge] Move processing of API data structures to sgen-bridge.c, for sgen-new-bridge.c.
[sgen-bridge] Also separate old bridge into pre- and post-callback.