first pass at using new keys

It compiles. It sorta works. Several subcommands are FIXME marked and
broken, because things that used to accept separate --backend and --key
params need to be changed to accept just a --key that encodes all the key
info, now that there is metadata in keys.

Upgrade.hs

@@ -13,6 +13,7 @@ import Control.Monad.State (liftIO)
 import Control.Monad (filterM, forM_)
 import System.Posix.Files
 import System.FilePath
+import Data.Maybe
 
 import Content
 import Types
@@ -74,7 +75,7 @@ getKeysPresent0' dir = do
 		else do
 			contents <- liftIO $ getDirectoryContents dir
 			files <- liftIO $ filterM present contents
-			return $ map fileKey files
+			return $ catMaybes $ map fileKey files
 	where
 		present d = do
 			result <- try $