add CredPair cache

Note that this does not yet use SecureMem. It would probably make sense for
the Password part of a CredPair to use SecureMem, and making that change
is better than passing in a String and having it converted to SecureMem in
this code.

Assistant/Types/CredPairCache.hs

@@ -0,0 +1,18 @@
+{- git-annex assistant CredPair cache.
+ -
+ - Copyright 2014 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Assistant.Types.CredPairCache where
+
+import Types.Creds
+
+import Control.Concurrent
+import qualified Data.Map as M
+
+type CredPairCache = MVar (M.Map Login Password)
+
+newCredPairCache :: IO CredPairCache
+newCredPairCache = newMVar M.empty