Add race helpers

[alfonsogarciacaro]

See MDN. We could have something like:

let race2 (pr1: 'T1) (pr2: 'T2): Promise<Choice<'T1, 'T2>>
let race3 (pr1: 'T1) (pr2: 'T2) (pr3: 'T3): Promise<Choice<'T1, 'T2, 'T3>>
..