(Maybe?) Validate the share ciphertext segments against the "crypt text tree"

The URI extension block contains the root hash of a merkle tree where the leaf nodes are hashes of the ciphertext segments. It's not clear to me why you would need to check the blocks against their hashes and the segments against their hashes. Maybe "defense in depth" (seems like a stretch to me) or to catch hardware-induced bitflips or bugs in the ZFEC implementation?

Sub-task of #16