Auto merge of #3170 - ebfull:sapling-merkle-tree, r=ebfull

Auto merge of #3170 - ebfull:sapling-merkle-tree, r=ebfull

Sapling merkle tree implementation

Closes #3056.

Please also review https://github.com/zcash/librustzcash/pull/8

This PR:

1. Introduces ZCSaplingIncrementalMerkleTree using Pedersen hashes.
2. Adds support for Sapling anchors into consensus rules. (Adds commitments, checks anchors are correct, handles block (dis)connects, etc.)
3. Handles mempool eviction for obsolete anchors.
4. Enforces correctness of block's Sapling root field
5. Changes miner to correctly apply the Sapling root to the block header
6. Handles mempool consistency checks for anchors