Improve API to access inner jubjub-point in PublicKey

[moCello]

Summary

Find a better way to access the underlying jubjub-elements than with as_ref (e.g. introducing an inner method).

Originally posted by @ureeves in dusk-network/phoenix#157 (comment)

Adding my two cents here. I think having a StealthAddress::address function is a bit... redundant to say the least, and confusing in the worst case. That said, I also understand the stealth_address.note_pk().as_ref() is unsatisfactory.

Maybe we should improve the access to the underlying JubJubScalar in NotePublicKey in a future PR?