Add option -j-gen-modules to generate JSON generic submodules → #416

[smondet]

PR checklist

• New code has tests to catch future regressions
• Documentation is up-to-date
• CHANGES.md is up-to-date

Cf. interesting bit in the tests:

atd/atdgen/test/test_atdgen_main.ml

Lines 664 to 676 in 923489d

	let test_generic () =
	let module Stringables = struct
	module type Mono = sig
	type t
	val of_string: string -> t
	val to_string: ?len: int -> t -> string
	end
	end in
	let test_involution (module M : Stringables.Mono) name input =
	let x = M.of_string input in
	let s = M.to_string x in
	Alcotest.(check string) (sprintf "involution-%s" name) input s;
	in

[mjambon]

Can you specify what we should expect here and/or add it to the docs?