Make pipe-first work with uncurried functions

[yawaramin]

Currently, this:

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

Does not work, with the error:

We've found a bug for you!
OCaml preview 2:15

This is an uncurried BuckleScript function. It must be applied with a dot.

Like this: foo(. a, b)
Not like this: foo(a, b)

This guarantees that your function is fully applied. More info here:
https://bucklescript.github.io/docs/en/function.html#solution-guaranteed-uncurrying

As we are placing more emphasis on uncurried syntax in BuckleScript, I think it makes sense to have the pipe-first operator work with uncurried functions as well.

[bobzhang]

I suggest this being done on the reason side in the parsing stage instead of relying on ppx.
cc @IwanKaramazow @anmonteiro

[bobzhang]

moved into reasonml/reason#2569