Initiate ortac-example package

[n-osborne]

The ortac-examples package aims at centralising specifications and ortac-generated code based on those specifications.

The first added example is the lwt_dllist librarie. This example display how to handle extension of the STM.ty type.

While writing the specification, I've stumbled upon #171 and #172.

[shym]

Nice work!
I’ve put a couple of suggestions as commits on my branch lwt-dllist-bis (and on (n-osborne#21)).

Other suggestions:

• I would have preferred it if ocamlformat had been disabled on lwt_dllist_spec.mli to make it easier to compare with the upstream version (it would also help maintain it)
• I expected that one of the take_*r functions would have a specification reading match Sequence.length (old s.contents) with 0 -> ... to illustrate that matching on integers is now supported :-) (it would also avoid duplicate calls to Sequence.length in take_opt_r)
• the dune rule to generate the lwt_dllist_tests would be nice there, even if only commented (to be honest, I would suggest to post-process them with a small script to inject the hand-written part :-)

[shym]

Oh! last suggestion: we are in the ortac repository, so I would prefer the directory to be named examples, I feel annoyed by the redundancy :-)

[shym]

I’ve reviewed the new changes: well done! LGTM!