OxCaml modes

[314eter]

Adds support for OxCaml modes and modalities.

The main difficulty was to get modes to work with function types. The left side of a function_type can only be a _simple_type, not a _type. But if the parser encounters let x : t @, it needs to choose if t is a _simple_type or _type, since it can't backtrack later. But depending on if it continues with let x : t @ mode = _ or let x : t @ mode -> t = _, the correct choice is different.

The easy solution would be to make the left side of function_type a _type, and use precedences to make sure everything is still parsed correctly. That almost works, except for

module M : T with type t = t as 'a -> t

which will then be parsed as T with type t = (t as 'a -> t) instead of (T with type t = t as 'a) -> t. I couldn't find a way to avoid that.

The alternative solution is to inline _type. That works, but it needs to be inlined in a lot of places, which results in a lot more states. This PR doubles the parser in size.

The other issue was polymorphic types. I had to refactor them a bit to be able to resolve conflicts like let x : 'a . 'a @ mode -> t = _. This has an impact on the resulting tree-sitter AST.

[314eter]

I reduced the number of states by adding conflicts instead of inlining rules to avoid them. I usually prefer to keep the grammar conflict free, but here it's not worth the increased complexity.

[tkoukpari]

do we want type_variable dropped?

[314eter]

The _ in type _ t = ... is not really a type variable, it's the absence of a type variable. I tried to make the parsing of _ a bit more consistent across the whole grammar, but completely ignoring it here is maybe not the best solution.