Fuzzer fixes

[314eter]

Some issues found by ocamlgrammarfuzzer:

Include functor

As mentioned in #138 (comment), the include functor support breaks some syntax that was previously valid but rejected by the type checker. Not really an issue in practice, but it's possible to support both by introducting a conflict.

Open module in signatures

Until now, the open_module rule in structures and signatures was the same. But in reality they are slightly different, and it's not possible to support both with one rule. So there is now a separate open_module_signature.

Module type constraints

Weird behaviour by the OCaml parser:

module type T = T with module type T = T -> T
module type T = T with module type T := T -> T

The first one is parsed as (T with module type T = T) -> T, the second one as T with module type T := (T -> T). I don't understand how that's possible (the rules look identical), but we should do the same.

Range patterns

Range patterns were not supported everywhere they should be. This is accepted now:

let f 'a' .. 'z' = ()

Extra constructors

Until now, boolean literals (true, false) and unit (()) were parsed as constants, but the OCaml parser handles them as special constructors. This is now accepted (but please don't):

type t = true of int
type 'a l = [] of 'a list

let f [] [] = true 0

The downside is that [] is not parsed as a list_expression anymore but as the empty_list constructor.

[let-def]

Very nice!

The weird difference between the two module type constraints (parsed as (T with module type T = T) -> T and module type T := (T -> T)) are (likely) due to the following precedence directives (from upstream parser):

%right    COLONEQUAL                    /* expr (e := e := e) */
%right    MINUSGREATER                  /* function_type (t -> t -> t) */
%left     EQUAL   /* expr (e OP e OP e) */

And this is probably accidental. I think it is worth reporting an issue on https://github.com/ocaml/ocaml, at least to raise awareness about this corner-case.

[314eter]

I updated the module type constraints fix according to ocaml/ocaml#14978.

[314eter]

Added some more fixes for issues found while fuzzing OxCaml syntax.