rename key2file and file2key

What these generate is not really suitable to be used as a filename,
which is why keyFile and fileKey further escape it. These are just
serializing Keys.

Also removed a quickcheck test that was very unlikely to test anything
useful, since it relied on random chance creating something that looks
like a serialized key. The other test is sufficient for testing what
that was intended to test anyway.

Crypto.hs

@@ -161,7 +161,7 @@ type EncKey = Key -> Key
  - on content. It does need to be repeatable. -}
 encryptKey :: Mac -> Cipher -> EncKey
 encryptKey mac c k = stubKey
-	{ keyName = encodeBS (macWithCipher mac c (key2file k))
+	{ keyName = encodeBS (macWithCipher mac c (serializeKey k))
 	, keyVariety = OtherKey $
 		encryptedBackendNamePrefix <> encodeBS (showMac mac)
 	}