Replace camlp5 with ppx_parser

[tobil4sk]

We've had issues with camlp5 on windows as it doesn't get much maintenance anymore (and still relies on Unix configure/makefiles rather than dune).

This PR is an attempt to port our parser to ppx_parser, which is a reimplementation of camlp-stream parsers with ppxlib preprocessors which are much better maintained. It was missing a couple features but I've manged to upstream these:

• Allow creating parser from match expression NielsMommen/ppx_parser#4
• Allow binding stream to variable NielsMommen/ppx_parser#5

We also need this patch to allow it to install with ocaml 4.08, so for now I've set it to build with my fork:

• Relax constraint on OCaml version NielsMommen/ppx_parser#6

ppx_parser is based on camlp4 which, unlike camlp5, did not perform automatic left-factorization. We've avoided this problem with manual refactoring: #11859.

Another minor difference is that when clauses are not supported within rules, only outside of them, so the clause can only include variables bound in the first element of the rule. This is not a big issue after the refactoring that was already required.

Other differences are purely syntactic.

[Simn]

@yuxiaomao Could you check if this works with your codebases? And if it does, could you check the --times before/after to make sure we don't have a parsing performance regression?

[yuxiaomao]

All projects compile (except one but it's already failing with development), and I can't notice significant performance change compared to development 5174e89 .
(For some projects I think development has slightly worse performance in parsing compared to 7f3dd53 , but that's out of the scope of this PR)

[yuxiaomao]

Note that when testing the compilation with one project (without compilation server), the first compilation is significant slower than usual (+50% total time, from 30s to 45s). Can't reproduce and difficult to say if it's related to this PR or not x)

[Simn]

I'm trying to do my local setup for this and will log everything I come across. With ppx_parser 0.1.1 installed I'm currently running into this error:

File "src/syntax/grammar.ml", lines 47-49, characters 8-14:
47 | ........match%parser s with
48 |            | [ (sep2,_); [%let l = psep_trailing sep f] ] when sep2 = sep -> v :: l
49 |            | [ ] -> [v]
Error: function expected

Edit: Right, as Yuxiao had already told me I need to run opam pin ppx_parser https://github.com/tobil4sk/ppx_parser.git#relax-ocaml-constraint for the time being. With that being done compilation just works!

[Simn]

@kLabz Could you check if this works for you as well?

I'm pretty eager to merge this quickly, which may or may not be because I nuked my local camlp5 setup and now can't compile Haxe on any other branch anymore...

[kLabz]

Can do that in 1~2 hours

[Simn]

I've removed most of the [%s s] because I don't like how this looks now. There are 23 occurrences left, maybe we should just go all the way and get rid of them. It's easy to do by changing the relevant functions from and name args = function%parser to and name args s = match%parser s with.

[Simn]

I'll go ahead and merge this now so that we can move forward. There might be some adjustments to make, but we can always do that later.

Thank you SO MUCH for looking into this, getting rid of camlp5 has been on my bucket list for a while now.

[RblSb]

pin-depends should be updated to use base git repository (not a fork) instead of current removed PR branch

[kLabz]

that has been done already