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 can't think of a reason you _wouldn't_ want to bind when discharging all the macaroons, but this behavior should at least be documented in the godoc comment.

Would binding twice break things? I have a feeling it would, I'd have to go look at the impl of Macaroon.Bind.