This PR changes the string interpolation procedure to omit redundant
empty parts. For example `s!"{1}{2}"` previously elaborated to `toString
"" ++ toString 1 ++ toString "" ++ toString 2 ++ toString ""` and now
elaborates to `toString 1 ++ toString 2`.
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.)
@Kha I wrote this extended test using the new frontend. The new
frontend worked great. I only had a minor issue with `#exit`.
The example uses `let rec`, parsers, macros, lift-method notation, etc.
It implements the new parser `strInterpolant p` for string
interpolation. The parser `p` is used to process the elements inside
`{...}`. Then, I use this parser to implement the macro `toString!`.
Example: `toString! "1+2 = {1+2}"` produces `"1+2 = 3"`.
Even the new "lift-method" notation works inside the string interpolant.
```lean
def g (x : Nat) : StateRefT Nat IO Nat := do
modify (· + x);
get
def ex : StateRefT Nat IO Unit := do
IO.println $ toString! ">> hello {(<- g 1)}";
IO.println $ toString! ">> world {(<- g 1)}";
pure ()
```
