Rename ak to ak_P in the circuit impl

[str4d]

The SpendValidatingKey type is designed such that by construction, the byte encodings of ak and ak_P are identical. However, once we expose the underlying point representation, we should use the names of the individual types to match the protocol spec. So the following places in the circuit code should use ak_P:

orchard/src/circuit.rs

Lines 365 to 371 in 99b767a

	// Witness ak.
	let ak: Option<pallas::Point> = self.ak.as_ref().map(|ak| ak.into());
	let ak = NonIdentityPoint::new(
	ecc_chip.clone(),
	layouter.namespace(|| "ak"),
	ak.map(|ak| ak.to_affine()),
	)?;

orchard/src/circuit.rs

Lines 593 to 594 in 99b767a

	// [alpha] SpendAuthG + ak
	let rk = alpha_commitment.add(layouter.namespace(|| "rk"), &ak)?;

And then here, instead of implicitly creating ak from ak_P, we should add a let ak = ... above the rivk definition:

orchard/src/circuit.rs

Lines 606 to 612 in 99b767a

	let rivk = self.rivk.map(|rivk| rivk.inner());
	commit_ivk_config.assign_region(
	config.sinsemilla_chip_1(),
	ecc_chip.clone(),
	layouter.namespace(|| "CommitIvk"),
	*ak.extract_p().inner(),