bakery: implement CheckAny

Factor CheckAny out from httpbakery.Check. There's no
new behaviour here.

Also (and trivial) rewrite []*macaroon.Macaroon to macaroon.Slice throughout
and change the argument order of httpbakery.Check.

I'd consider creating a type for `map[string]string`. godocs and helper methods on that might make for a more fluent readable API.