Quote and escape Observable strings and chars

[jmid]

When recently using QCheck's function generation support, I noticed an issue with its printing of chars and strings.
Consider these two examples:

open QCheck

let test_char =
  Test.make
    (triple char char (fun1 Observable.char bool))
    (fun (c1,c2,f) -> Fn.apply f c1 = Fn.apply f c2)

let _ = QCheck_base_runner.run_tests ~verbose:true [test_char]

let test_string =
  Test.make
    (triple string string (fun1 Observable.string bool))
    (fun (s1,s2,f) -> Fn.apply f s1 = Fn.apply f s2)

let _ = QCheck_base_runner.run_tests ~verbose:true [test_string]

resulting in output such as:

generated error fail pass / total     time test name
[✗]    1    0    1    0 /  100     0.0s anon_test_1

--- Failure --------------------------------------------------------------------

Test anon_test_1 failed (15 shrink steps):

('a', '\142', {� -> true; _ -> false})
================================================================================
failure (1 tests failed, 0 tests errored, ran 1 tests)
generated error fail pass / total     time test name
[✗]    2    0    1    1 /  100     0.0s anon_test_2

--- Failure --------------------------------------------------------------------

Test anon_test_2 failed (27 shrink steps):

("a", "", { -> false; _ -> true})

Note the not-so-helpful function domain printing: � -> true and -> false.

This PR therefore offers a quickfix to these two printers, aligning them with the arbitrary printers, and thus offering a uniform print out:

generated error fail pass / total     time test name
[✗]    1    0    1    0 /  100     0.0s anon_test_1

--- Failure --------------------------------------------------------------------

Test anon_test_1 failed (18 shrink steps):

('a', '_', {'_' -> false; _ -> true})
================================================================================
failure (1 tests failed, 0 tests errored, ran 1 tests)
generated error fail pass / total     time test name
[✗]    1    0    1    0 /  100     0.0s anon_test_2

--- Failure --------------------------------------------------------------------

Test anon_test_2 failed (34 shrink steps):

("a", "", {"" -> false; _ -> true})
================================================================================
failure (1 tests failed, 0 tests errored, ran 1 tests)

Looking further,

• I was annoyed by having to duplicate the ad-hoc printers from arbitrary one more time, rather than use the Print module, supposed to be used for this
• I also noticed that QCheck2.Print.char and QCheck2.Print.string already behave like the above, creating a QCheck.Print / QCheck2.Print mismatch for chars and strings 😱

I'll therefore file a sister-PR with a breaking change to QCheck.Print.{char,string} restoring uniformity and order to the universe... ⭐ ...then we discuss the preferred solution and whether anyone's (expect) tests may be broken by it.

[jmid]

That the Print.{char,string} combinators do not properly quote and escape is a genuine problem

• that we've had to work around in multicoretests and
• that is now showing up in Ortac's QCheck-STM plugin Add function generation support  ocaml-gospel/ortac#277

Since #297 does not break any reverse dependencies on the opam repo, I plan to close this one and go with #297 unless I receive any objections.

[jmid]

Closing this in favour of #297