this is the simplest of the constructions to be ported from C++ to Lean,
so I’ll PR this one first.
This begins to put each construction into its own file, as it was the
case with C++.
For validation I developed this in a separate repository at
https://github.com/nomeata/lean-constructions/tree/fad715e
and checked that all `.recOn` declarations found in Lean and Mathlib are
identical (per `==`) to the ones produced by the C code.
Many of our tests in `tests/lean/run/` produce output from `#eval` (or
`#check`) statements, that is then ignored.
This PR tries to capture all the useful output using `#guard_msgs`. I've
only done a cursory check that the output is still sane --- there is a
chance that some "unchecked" tests have already accumulated regressions
and this just cements them!
In the other direction, I did identify two rotten tests:
* a minor one in `setStructInstNotation.lean`, where a comment says `Set
Nat`, but `#check` actually prints `?_`. Weird?
* `CompilerProbe.lean` is generating empty output, apparently indicating
that something is broken, but I don't know the signficance of this file.
In any case, I'll ask about these elsewhere.
(This started by noticing that a recent `grind` test file had an
untested `trace_state`, and then got carried away.)
