bakery: remove Request type.

httpbakery: remove Service type. Add cookie helpers.

Also change bakery.DischargeAll so that it returns a slice suitable
for putting in a cookie.
I hope you don't mind, but I think it's best to leave this change for another PR. This one's already quite big, and making that change is not insignificant. I'm not sure that the situation is as bad as you paint it, because I don't think it will be useful for clients to act differently depending on whether macaroons have their discharges attached or not. It's the client's responsibility to add discharges - they can tell before sending them whether they need discharges. The verification logic can fail in all kinds of ways, and the intention is to respond to any of them by creating a new macaroon to be discharged. So the main reason for returning all errors is for informational purposes only AFAICS.