Add select future combinator

[igalshilman]

with that we can write code like that:

 match await select(result=ctx.promise("verify.payment"), timeout=ctx.sleep(TIMEOUT)):
        case ['result', "approved"]:
            ctx.set("status", "payment approved")
            return { "success" : True }
        case ['result', "declined"]:
            ctx.set("status", "payment declined")
            raise TerminalError(message="Payment declined", status_code=401)
        case ['timeout', _]:
            ctx.set("status", "payment verification timed out")
            raise TerminalError(message="Payment verification timed out", status_code=410)