RollingMerkleTree is a much more memory efficient way of calculating the
merkle root of a tx commitment inside the bitcoin block header. The
current way of calculating the merkle root allocates 2*N elements. With
the RollingMerkleTree, we are able to reduce the memory allocated to
log2(N).
This results in significant memory savings (99.9% in an average block),
allowing for a faster block verification.
