This PR modifies the pretty printing of raw natural number literals; now
both `pp.explicit` and `pp.natLit` enable the `nat_lit` prefix. An
effect of this is that the hover on such a literal in the Infoview has
the `nat_lit` prefix.
Amendment to RFC #3021: In the reference-level explanation, now it
should read
> When `pp.natLit` and `pp.explicit` are false, then the `nat_lit n`
expression delaborates as `n`, and otherwise it delaborates as `nat_lit
n`.
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.)
