These changes pay particular attention to API endpoints, to try to
ensure that any rustdocs generated are detailed and sensible. A good
next step, eventually, might be to include doctest examples, as well.
For the moment, it's not clear that they would provide a lot of value,
though.
In addition, this does a couple refactors to simplify the code base in
ways that make things clearer or, at least, briefer.
This change adds `Arbitrary` instances to the key IR data types (both as
syntax and as native IR), as well as evaluator functions for both. This
way, we can ensure that the evaluation of one version of the IR is
observationally equivalent to another version of the IR, or even a later
IR. It also adds a similar ability through both static file compilation
and the JIT, to ensure that the translation through Cranelift and our
runtime works as expected.
This actually found a couple issues in its creation, and I hope is
helpful extensions into more interesting programs.
