Misc. preparations for multi-core support.

• Fix xdot[false].
• Basic operators in RT backend.
• CCP propagate dynamic constant constants.
• Add some debugging utility to hercules_rt.
• Add sub-edits to Forkify, ForkChunk, and ForkSplit.
• Return new fork-join node IDs from ForkSplit, and label those nodes in the pass manager.
• New fork-join test to be used in the future for developing multicore support.