Orchard note commitment tree test vectors

[dconnolly]

Motivation

We need to check conformance between our implementation of Orchard note and value commitments, note commitment trees, and anything else that uses the Sinsemilla hash.

Specifications

Fixed a bug about not encoding the j in S(j) as a 10 bit in 4 little endian bytes, we just were just encoding it with 2 bytes.

https://zips.z.cash/protocol/protocol.pdf#concretesinsemillahash

Solution

Bring in several test vectors for Orchard note commitment tree empty roots, and Sinsemilla test vectors, from the zcash-hackworks library, and cross-checked against existing implementations in the ECC orchard crate.

Review

@conradoplg @upbqdn (if they have time)

Reviewer Checklist

• Code implements Specs and Designs
• Tests for Expected Behaviour

Follow Up Work

Partially addresses #2079,
related to #1287

[teor2345]

(I've removed jvff as a reviewer, so he can focus on network stuff.)

[conradoplg]

Looks good! 👍

There is just a small documentation fix