Deriver: support qcheck and qcheck2

[vch9]

The deriver should create QCheck and QCheck2 generators.
The choice could be given in the deriver's plugin name: [@@deriving qcheck] and [@@deriving qcheck2].

[jmid]

[@@deriving qcheck] and [@@deriving qcheck2].

Looks like good names to me!

[vch9]

I think qcheck and qcheck2 are fine when we only derive one thing. When #191 is done, what would the plugin qcheck derives?
We could have the library version as a prefix: [@@deriving qcheck.gen] and [@@deriving qcheck.shrinker]

[c-cube]

Would it make sense for [@@deriving qcheck] to also produce an arbitrary object in addition to gen + shrinker? The suffix versions would be used to derive only specific components.

[vch9]

Would it make sense for [@@deriving qcheck] to also produce an arbitrary object in addition to gen + shrinker?

Well, yes we can. However, there are two solutions:

• Derive generator and call QCheck.make on them
  -> PROS: this is really easy to implement
  -> CONS: arbitrary fields will be missing (e.g. printer)
• Specific derivation for arbitrary
  -> PROS: use built-in arbitrary (i.e. with printer, shrinkers etc).
  -> CONS: needs time

The suffix versions would be used to derive only specific components.

Oh yes