Expose extract_p and pallas_group_hash

[daira]

From ZcashFoundation/zebra#9426 :

Follow-up Work

The Sinsemilla library does not expose some of the functions needed to remove all of our code. So we've had to keep extract_p() and pallas_group_hash(). We might want to talk to the implementers to see if they'll expose them but it's not in the scope of this story.

TODO: figure out why these are needed in Zebra when they are not in zcashd.