crypto: secure key wrapping/unwrapping

crypto: secure key wrapping/unwrapping

This commit adds in the ability to use the WrappedKeyData from the
metadata package to wrap and unwrap cryptographic keys of any length.
This makes use of several cryptographic primitives:
	- Unsalted, SHA256-based HKDF for key stretching
	- AES256 in CTR mode for encryption
	- SHA256-based HMAC for authentication

Note that the key wrapping/unwrapping uses an "Encrypt then MAC" scheme
for doing authenticated unwrapping. This means we can detect if bogus
metadata has been given. This package also standardizes the length for
fscrypt's internal keys.

This CL is the first to add benchmarks, which can be run with:
	go test -bench=. ./...

Change-Id: I2e5fc23a8a8cc36b17ccb3f26f03edcaccc517e1