Improve performance of authorizing data commitment

[conradoplg]

Motivation

#2547 implements the authorizing data commitment specified in ZIP-244 which is the root of a Merkle tree of the auth digest of each transaction. (zebra-chain/src/block/merkle.rs, FromIterator for AuthDataRoot)

This tree is padded with leaves containing all-zeroes hashes to make the tree full. However that can be expensive.

We can either optimize that by pre-computing the roots of of fully-"empty" subtrees and using that (like note commitment trees do), or by making sure this is only computed once for each block and reused where needed.

Specifications

Designs

Related Work

Follow up to #2547

[teor2345]

We can re-open this if it turns up in CPU profiles.