*: Use sha-512 digest format

Per appc/spec#8, we should be using sha-512 hashes for speed
cas: tweaks and added tests

Mostly addressed comments in https://github.com/coreos/rocket/pull/209
- errors should start with lowercase
- split out consts for hash prefixes/lengths
- added tests for ResolveKey
- added test to check for unexpected goroutines
- revert to panicking if we get unexpected input to blockTransform
cas: drop unused afterTest func
*: switch cas using first half of sha512

This changes the CAS to use only the first half of a SHA512 when
creating the keys to store images on disk. Similarly, rkt stage1 uses
only the first half of the sum when creating the directories for each
app image.

This is exposed slightly to stage0/run because it attempts to verify the
hash of the image. This should be revisited+cleaned up.
Merge pull request #290 from jonboulle/rckclmbr-sha512

*: sha256 -> sha512