Validate the crypttext hash tree and the segments

Fixes #21 (closed)

This does add automated tests for validation of supporting pieces (validating the "crypttext hash tree" for example).

This does not add automated tests for the validation of the ciphertext hashes.

I can see no straightforward way to test the actual ciphertext hash validation as invalid ciphertext and invalid ciphertext hashes are excluded by earlier validation. Producing invalid ciphertext or ciphertext hashes on-demand is just not possible during normal operation.