Improve the syntax of ppx rewriters and flags

[ghost]

I never really liked the current syntax. This PR improves the syntax of (pps ...) as follow:

• old syntax: (pps (ppx1 -arg1 ppx2 (-foo x)))
• new syntax: (pps ppx1 -arg ppx2 -- -foo x)

[rgrinberg]

I'm not such a fan of using the special -- syntax here tbh. -- looks like something that will be passed as a literal argument which isn't really the case here.

Also, did the following work before:

(pps (ppx1 (-foo x) ppx2 -arg22))

If it did, how would it get translated?

[ghost]

It worked and translates to: (pps ppx1 ppx2 -- -foo x -arg22). Regarding the use of --, I see your point but it still looks like an improvement to me compared to the current syntax.

[rgrinberg]

Yeah, I agree this is an improvement. I suppose that to be fully explicit we'd need something like a separate args fields for the pps constructor. This might seem like a heavy approach but I think it could pay off for customizing the :standard set of flags vs the :standard set of pp's via the env stanza.