Serialize state machines in the torrc

[asn]

Figure out how to serialize state machines in the torrc and consensus to make it easire to specify state machines for researchers (and for common users).

Perhaps it's fine to do this after 0.4.0 since the WTF-PAD code in 0.4.0 is mainly for research purposes and not inteded to be used by a wide audience. The researchers can perhaps write code of their own that instantiates the padding machines if we write a good enough doc to do so.

[asn]

from todo file:

- circpad_machine_validate() function to sanity-check histograms loaded from
  consensus/torrc (can also be used to help guide a GA).
  - Check bin construction
  - no type overflow (start_usec + range_sec, etc)
  - no conflicting state transitions (or overlap with cancel events)
  - no use of both histograms and iat_dist
  - at least two histogram bins
  - min_hop vs target_hop

[mikeperry]

(Figure 5 days for torrc serialization + testing + optimization and another 5 for consensus work and monitoring).

[mikeperry]

Also, back-and-forth for review will take 3-4 days, I bet.

[mikeperry]

Separated out the consensus piece into #29102

[mikeperry]

I'm lowering the priority of this based on engineering cost. I think researchers will likely need to recompile their relays' tors for some time still, while working on this stuff.

If anyone external to Tor will find this useful, let me know.