Support user-defined scheduler

[yihozhang]

To support user-defined schedulers, this PR does a few things

• Expose a safe set of timestamp APIs
• Small tweak to EGraph::run_actoins to make it more ergonomic
• Allow associating rules with properties using the :prop "prop-name" literal syntax. This is useful for passing user-defined scheduling parameters to the scheduler.
• Fixed a bug that ignores user-provided rule names (closes #323) and a bug that creates rules with duplicate names when semi-naive transformation
• Add a with-scheduler keyword to the schedule DSL
• Move scheduling-related functions to a separate scheduler.rs
• Exposed scheduler as a trait similar to egg.
• Implemented an example Backoff scheduler that has exact the same behavior as egg's backoff scheduler on the math microbenchmark

Currently, the following command is supported:

(run-schedule
  (repeat 5
    (with-scheduler (simple)
      (repeat 5 (run analysis)))
    (with-scheduler (backoff)
      (repeat 5 (run optimization))))