More direct encoding of uncurried function and type definitions

[cristianoc]

Instead of using the @bs attribute in the parser/printer followed by a PPX in the compiler: use the direct encoding.

• for types Syntax: process uncurried types explicitly in the parser/printer. #5784 Fix parsing type annotations starting with 'a in uncurried mode. #5822
• for definitions Syntax: process uncurried function declarations explicitly in the parser/printer #5794

// type t0 = (.) => int
type t0 = Js.Fn.arity0<int>

// let f0: t0 = (.) => 3
let f0: t0 = {Js.Fn.\"I0": () => 3}

// type t1 = (.int) => int
type t1 = Js.Fn.arity1<int => int>

// let f1: t1 = (.z) => z+1
let f1: t1 = {Js.Fn.\"I1": z => z + 1}

[bobzhang]

note the direct encoding would not trigger any regression.
so that we can add the direct encoding first and remove the ppx later (if needed)