Fix QCheck2.Gen.small_string impl to match interface and documentation

[jmid]

This PR updates the QCheck2.Gen.small_string to match the interface and documentation following my finding in #153:

valsmall_string : ?gen:chart -> stringt(** Builds a string generator, length is {!small_nat}. Accepts an optional character generator (the default is {!char}). Shrinks on the number of characters first, then on the characters.*)

[vch9]

👍

[jmid]

It is nice to align the .ml and .mli for consistency.
However, as far as I can tell (see examples and discussion in #162) this still means that the parameter is defacto labelled - and thus required. Merging this fix does not change that - meaning the documentation+interface should be updated 😬🤷‍♂️
As such I'm hesitant with the merge, because we will have to make some additional changes anyway...

I'd therefore like your input on the discussion in #162@vch9@sir4ur0n@c-cube
(I'd also be grateful if one of you would confirm by cloning, running dune utop and pasting the examples)

[sir4ur0n]

While it doesn't solve the "optional mandatory" argument design issue you mentioned, it doesn't hurt in any way, so it's still positive 👍

[jmid]

Closing as this is addressed in #245