v0.2.1

Implements Synod 0.2. synod run simulates a program deterministically and writes its trace.

Added

• synod run <folder> checks the folder's main file and runs it in the
  deterministic simulator: simulated time, nodes with crash and restart,
  a network with latency, loss, partitions and targeted drops, per-node
  disk, and the always invariants checked after every step. It writes
  out/trace.jsonl (synod-trace/1) and prints one status line. The same
  program and seed give a byte-identical trace on every platform.
• --seed n (or --seed=n, a whole number from 0 to 2^53) replaces the
  seed of the run; it wins over sim.Seed. Without either the seed is 1.
• --trace file chooses the trace file (parents are created); --trace -
  writes the trace to stdout and the status line to stderr.
• --summary prints a compact text summary of the run (steps that
  recorded facts, invariant results, final state of every node) instead of
  the status line, for agents.
• Every native phrase of the std catalog works: sim, net, time,
  random, lists, maps, math, text, disk. The look phrases and
  the observers wait for a later release; --scene, --pace, --mode,
  --watch and --json-events are recognized and exit 2 with
  not implemented yet.
• Run-time errors, reported as the last sim.Failed line of the trace,
  failed at <time>: <pos>: error[<code>]: <message> and exit 1:
  missing-key, index-range, division-by-zero, nobody, disk-type,
  time-limit (10 minutes of simulated time), bad-argument,
  number-range and run-limit (steps at one instant, call depth,
  collection size).
• Exit codes of synod run: 0 for a run that ended (also when an always
  broke, which is a result, not an error), 1 for diagnostics or a failed
  run, 2 for usage and I/O errors.

Changed

• sim.Seed and math.Hash are specified completely in the std catalog
  (domain and precedence of the seed; the exact FNV-1a based hash).
• Demo seeds that did not tell their story under the final simulator
  semantics were changed (gossip 15, paxos 105, twopc 1, cassandra 4) and
  the ring demo places other keys; see "Notes for implementers" in the
  examples.