async gets formatted away for functions with labeled arguments

[cknitt]

let f = async (a, b) => a + b

let f = async (~a, ~b) => a + b

gets reformatted to

let f = async (a, b) => a + b

let f = (~a, ~b) => a + b

[cknitt]

Also

let f = async a => a

gets formatted to

let f = async (a) => a

Is this intentional? Because for non-async functions we are not adding parentheses automatically.

[mununki]

Fixed #677