Syntax: process uncurried types explicitly in the parser/printer.

[cristianoc]

Moved from rescript-lang/syntax#717

[cristianoc]

The CI fail is a good example of catching issues earlier now that the syntax repo has moved.

[cristianoc]

One issue with this is that one loses the ability to convert uncurried types from .ml e.g. in type u0 = ?x:int -> y : string -> int [@bs] the uncurried attribute is currently ignored.

So either one loses that ability, or a duplicate printing mechanism is required where the ast with @bs and the one with arity are both supported for printing.