switch to the strict state monad

I had not realized what a memory leak the lazy state monad could be,
although I have not seen much evidence of actual leaking in git-annex.
However, if running git-annex on a great many files, this could matter.

The additional Utility.State.changeState adds even more strictness,
avoiding a problem I saw in github-backup where repeatedly modifying
state built up a huge pile of thunks.

Utility/State.hs

@@ -0,0 +1,26 @@
+{- state monad support
+ -
+ - Copyright 2012 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Utility.State where
+
+import Control.Monad.State.Strict
+
+{- Modifies Control.Monad.State's state, forcing a strict update.
+ - This avoids building thunks in the state and leaking.
+ - Why it's not the default, I don't know.
+ -
+ - Example: changeState $ \s -> s { foo = bar }
+ -}
+changeState :: MonadState s m => (s -> s) -> m ()
+changeState f = do
+	x <- get
+	put $! f x
+
+{- Gets a value from the internal state, selected by the passed value
+ - constructor. -}
+getState :: MonadState s m => (s -> a) -> m a
+getState = gets