WIP: Allow bit-reproducible results with 1 main and 1 runner thread

A variety of mostly minor changes which allow reproducible results with a single runner. A couple one-line changes, where the original code is safe but hinders this goal, are enabled only by a DETERMINISTIC_SCHEDULE preprocessor flag. If you'd like to avoid preprocessor proliferation, these changes can be removed easily enough (but are probably worth documenting).

Resolves #58.