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.
I'd just say:

    // PublicKeyForLocation implements bakery.PublicKeyLocator.

I have some more thoughts on this that I'll put in the main comment for this review.