crypto: tests, errors, and descriptor computation

crypto: tests, errors, and descriptor computation

This changes the crypto package so it now builds in light of the changes
to the util and metadata package. This commit also improves the error
handling, adds tests, and makes it so recovery keys now correspond to
Policy keys (as they are used to recover a directory in the absence of
any metadata).

The only feature addition here is the ability to compute descriptors.
For backwards compatibility, we keep the same descriptor algorithm used
before (double SHA512).

Change-Id: Ia2b53c6e85ce65c57595e6823d3c4c92219bc8dc