add inode cache to the db

Renamed the db to keys, since it is various info about a Keys.

Dropping a key will update its pointer files, as long as their content can
be verified to be unmodified. This falls back to checksum verification, but
I want it to use an InodeCache of the key, for speed. But, I have not made
anything populate that cache yet.

Database/Keys/Types.hs

@@ -0,0 +1,14 @@
+{- Sqlite database of information about Keys, data types.
+ -
+ - Copyright 2015 Joey Hess <id@joeyh.name>
+ -:
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Database.Keys.Types (
+	DbHandle(..)
+) where
+
+import qualified Database.Handle as H
+
+newtype DbHandle = DbHandle H.DbHandle