Make pipe-first work with uncurried function

[sikanhe]

Old issue in reason repo: reasonml/reason#2569

cc @yawaramin

[IwanKaramazow]

Moved to rescript-lang/rescript#4629

From a syntax perspective we seem to parse the expected ast:

let f = (. x, y) => x + y
let x = 1->f(. 2)

is the same as

let f = fun [@bs] a b -> a + b
let x = 1 |. f 2 [@bs]