Refactor third-party caveat encoder/decoder to caveatid package.

Remove non-public-key third-party caveatid encoding for now (this can always be added
back later as an alternative implementation of bakery.CaveatId{Encoder,Decoder}).
Updated httpbakery & tests accordingly.
Also, I'd be tempted to define:

    type PublicKeyLocator interface {
        PublicKeyForLocation(loc string) *PublicKey
    }

instead of using the function type as primary.