This commit changes all the internal import paths from `fscrypt/foo` to
`github.com/google/fscrypt/foo` so that it can be built once we release
externaly. The documentation in README.md is updated accordingly.

Also, the README has a note noting that we do not make any guarantees
about project stability before 1.0 (when it ships with Ubuntu).

Change-Id: I6ba86e442c74057c8a06ba32a42e17f94833e280

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

This commit adds in the concept of recovery codes: human-readable
strings that contain the necessary information to rederive a
cryptographic key. These keys look like:
	73PZBXVP-DKJX7SKV-NNTFIC7A-QEGRPZUX-4K5ORRH2-MTKMKP3B-HFCA====

They are input or output directly to a io.Reader or io.Writer
respectively. This prevents the data from passing through unsecured
memory before it gets to its destination. Of course, if the provided
io.Reader or io.Writer is insecure, there is nothing we can do. In most
cases the provided io.Reader or io.Writer will be stdin or stdout. In
some rare cases you might want to pipe the output to another key.

This commit also adds tests and benchmarks for encoding/decoding
recovery codes. It also tests that encoding/decoding will fail in the
correct situations. A benchmark is also added to measure the effect of
locking the keys in memory.

Change-Id: Ifa0bc4c08582789785cf1cdd9a4acfe76c79534f