@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 ()
```
