This will be composed into depth-8-subtree-based computation of entire
sparse Merkle trees.

This is intended for comparison with the benchmarks from the previous
commit. This benchmark represents the theoretical perfect-efficiency
performance we could possibly (but impractically) get for computing
depth-8 sparse Merkle subtrees.

This commit ensures that `SparseMerkleTree::build_subtree()` can
correctly compose into building an entire sparse Merkle tree, without
yet getting into potential complications concurrency introduces.

Building on the previous commit, this commit implements a test proving
that `SparseMerkleTree::build_subtree()` can be composed into itself not
just concurrently, but in parallel, without issue.

This commit adds a new required method to the SparseMerkleTree trait,
to allow generic construction from pre-computed parts.

This will be used to add a generic version of `with_entries()` in a
later commit.

What the previous few commits have been leading up to: SparseMerkleTree
now has a function to construct the tree from existing data in parallel.
This is significantly faster than the singlethreaded equivalent.
Benchmarks incoming!