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.

Command/Find.hs

@@ -88,7 +88,7 @@ showFormatted format unformatted vars =
 
 keyVars :: Key -> [(String, String)]
 keyVars key =
-	[ ("key", key2file key)
+	[ ("key", serializeKey key)
 	, ("backend", decodeBS $ formatKeyVariety $ keyVariety key)
 	, ("bytesize", size show)
 	, ("humansize", size $ roughSize storageUnits True)