Add [@@deriving qcheck] support in interface files

[Niols]

Register a sig_type_decl expander for both qcheck and qcheck2 derivers, so that [@@deriving qcheck] in .mli files generates val declarations for generators (and arbitraries for QCheck).

For example:

type 'a t [@@deriving qcheck2]

produces:

val gen : 'a QCheck2.Gen.t -> 'a t QCheck2.Gen.t

Parametrized types are handled by adding one Gen.t arrow per type parameter. Sized generator signatures (gen_*_sized) are intentionally left out.

Closes #189

I hope this addresses the concerns raised in #286 — let me know if there is anything you would like to see improved.

[jmid]

Thanks a bunch for this! 🙏
I've only given it a brief skim for now. I will take the time for a proper review soonish... 🙂

[Niols]

Of course, take your time! It took me months to motivate myself to write this and months before to consider contributing, what's a few more months!

[Niols]

Hey there! Gentle ping, but there is still no rush :-)

[jmid]

Thanks a bunch for this - and sorry for the delay.

LGTM!

Minor nit: The comments regarding the arb-generator can perhaps state
a bit more clearly that this is strictly for QCheck. I've made a few suggestions
there, which I'll commit and then merge.

Thanks again! 🙏

[jmid]

I've also added a brief mention of the module signature support in the ppx_deriving_qcheck README file.

CI is failing on 32-bit workflows which is an unrelated issue #415

[Niols]

Thanks a lot! No worries about the timing, I know how it goes. Thanks for coming back to it :-)

IIUC I have nothing to do, but if you'll have me do something I'll happily oblige (but I don't touch a computer until mid-September at the earliest).