Design summary for the Haskell simulator

[nfrisby]

This Issue reifies the idea first stated in this comment on my first non-trivial PR #413 (comment).

This Issue is done when I write the first summary of the simulator as-is, before any of my changes. My subsequent PRs will amend that file.

This documentation is intended to aid:

• Someone trying to better understand which behaviors induced some simulation results, without having to read the code itself.
• Someone trying to enrich the simulator.
• Someone trying to compare and contrast the Haskell and Rust simulators' features and behaviors.

[nfrisby]

PR 427 (see link in GitHub timeline just above ^^^) has already discharged a big chunk of this Issue, but not yet the whole thing.

[nfrisby]

I pivoted PR 417 (see link in GitHub timeline just above ^^^) to flesh out the rest of the document.

Once that PR merges, I'll close this Issue. All of the remaining TODOs are either minor (eg "XYZ would be more realistic") or major ("explain the simulator's Praos logic"). The major ones would obviously be useful, but they seem too large to do right now, just for the sake of this issue---we'll create corresponding Issues when those stubbed explanations become relevant and/or urgent.