`QCheck.char` doesn't shrink (but `QCheck2.Gen.char` does)

[jmid]

I was surprised to discover this - illustrated by test char_is_never_abcdef.
QCheck reports a 'd' counterexample, while QCheck2 shrinks it to a:

Test char is never produces 'abcdef' failed (0 shrink steps): |	Test char is never produces 'abcdef' failed (1 shrink steps):

'd'							 |	'a'

This is a defect in QCheck.char IMO - users would expect it to shrink.

Originally posted by @jmid in #153 (comment)

[jmid]

We even have a QCheck.Shrink.char shrinker:

let char c yield =
 if Char.code c > 0 then yield (Char.chr (Char.code c-1))

It just isn't used by QCheck.char! 🤔 As a first version, we could consider redefining the char arbitrary as:

let char = make_scalar ~print:(sprintf "%C") ~shrink:Shrink.char Gen.char

Potentially it could be refined to shrink towards 'a' like QCheck2.

[jmid]

Fixed by #247