Call the key in desc-auth-ephemeral-key, KP_hs_desc_ephem

Proposed by @nickm in arti!999 (diffs)

(I think the spec is not 100% clear that hs_y and hs_Y are this key, rather than some other possible ephemeral keypair the HS might have, so please would the reviewer check that this is actually true.)

Also, use the proper names KP_hsc_desc_enc rather than inventing different local names. Again, I'm not 100% sure that this is the same key but I don't see what other key KP_hsc_desc_enc could be.