Port to ppxlib and OCaml 4.11 (continuation of #111 and #118)

[thierry-martinez]

This pull-request pursues the work that @rgrinberg and @kit-ty-kate started to port ppx_deriving_yojson to ppxlib and OCaml 4.11. See #111 and #118.

My commit includes only minor modifications for location handling. Previously, !Ast_helper.default_loc was used without being initialized: I tried to use more precise location informations when available.

[gasche]

This looks nice, thanks! (And thanks to @rgrinberg and @kit-ty-kate of course.)

One uninformed question: there is a deep change in the way locations are being handled, now they are threaded through the plugin as a new parameter. This is a new change you made. Can you explain why it is necessary? (I guess this is something about a difference in the way ppx_metaquot vs. ppxlib's metaquoter work?)

Is this something that should be documented for other authors of ppx_deriving plugins? (Do I correctly understand that this means that the ppxlib change has more severe backward-compatibility implications than we thought at first?)

[thierry-martinez]

there is a deep change in the way locations are being handled, now they are threaded through the plugin as a new parameter. This is a new change you made. Can you explain why it is necessary? (I guess this is something about a difference in the way ppx_metaquot vs. ppxlib's metaquoter work?)

Yes, ppxlib.metaquot expects loc to be bound in the environment, while ppx_tools.metaquot relied on !Ast_helper.default_loc: we already noted this change in ocaml-ppx/ppx_deriving#206 (comment) but it is worth documenting this for other authors.

[gasche]

Let me go ahead and merge this one as well. Thanks @thierry-martinez and @rgrinberg !

It is not completely clear when we will be ready to migrate all ppx_deriving plugins to the ppxlib approach (for example we found an upstream bug in the process, ocaml-ppx/ocaml-migrate-parsetree#97 , and I wonder if we should wait for it to be fixed; there are some compatibility issues that need ironing out or careful documentation). What @thierry-martinez proposed is thus to create pre-ppxlib branch to not delay the 4.11-compatible release process. This is what we are doing for now (master has ppxlib support, but is not quite release-ready, and pre-ppxlib is the boring, safe approach for yet another OCaml release).