function Promise_spread(value, onreturn, onthrow) {
    return Promise_return(value).spread(onreturn, onthrow);
}
Spreads the values of a promised array of arguments into the
fulfillment callback.
@param onreturn callback that receives variadic arguments from the
promised array
@param onthrow callback that receives the exception if the promise
is rejected.
@returns a promise for the return value or thrown exception of
either callback.