cache the serialization of a Key

This will speed up the common case where a Key is deserialized from
disk, but is then serialized to build eg, the path to the annex object.

Previously attempted in 4536c93bb2
and reverted in 96aba8eff7.
The problems mentioned in the latter commit are addressed now:

Read/Show of KeyData is backwards-compatible with Read/Show of Key from before
this change, so Types.Distribution will keep working.

The Eq instance is fixed.

Also, Key has smart constructors, avoiding needing to remember to update
the cached serialization.

Used git-annex benchmark:
  find is 7% faster
  whereis is 3% faster
  get when all files are already present is 5% faster
Generally, the benchmarks are running 0.1 seconds faster per 2000 files,
on a ram disk in my laptop.

Command/Info.hs

@@ -50,23 +50,23 @@ import qualified Command.Unused
 type Stat = StatState (Maybe (String, StatState String))
 
 -- data about a set of keys
-data KeyData = KeyData
+data KeyInfo = KeyInfo
 	{ countKeys :: Integer
 	, sizeKeys :: Integer
 	, unknownSizeKeys :: Integer
 	, backendsKeys :: M.Map KeyVariety Integer
 	}
 	
-instance Sem.Semigroup KeyData where
-	a <> b = KeyData
+instance Sem.Semigroup KeyInfo where
+	a <> b = KeyInfo
 		{ countKeys = countKeys a + countKeys b
 		, sizeKeys = sizeKeys a + sizeKeys b
 		, unknownSizeKeys = unknownSizeKeys a + unknownSizeKeys b
 		, backendsKeys = backendsKeys a <> backendsKeys b
 		}
 
-instance Monoid KeyData where
-	mempty = KeyData 0 0 0 M.empty
+instance Monoid KeyInfo where
+	mempty = KeyInfo 0 0 0 M.empty
 
 data NumCopiesStats = NumCopiesStats
 	{ numCopiesVarianceMap :: M.Map Variance Integer
@@ -82,9 +82,9 @@ instance Show Variance where
 
 -- cached info that multiple Stats use
 data StatInfo = StatInfo
-	{ presentData :: Maybe KeyData
-	, referencedData :: Maybe KeyData
-	, repoData :: M.Map UUID KeyData
+	{ presentData :: Maybe KeyInfo
+	, referencedData :: Maybe KeyInfo
+	, repoData :: M.Map UUID KeyInfo
 	, numCopiesStats :: Maybe NumCopiesStats
 	, infoOptions :: InfoOptions
 	}
@@ -512,7 +512,7 @@ reposizes_total :: Stat
 reposizes_total = simpleStat "combined size of repositories containing these files" $
 	showSizeKeys . mconcat . M.elems =<< cachedRepoData
 
-cachedPresentData :: StatState KeyData
+cachedPresentData :: StatState KeyInfo
 cachedPresentData = do
 	s <- get
 	case presentData s of
@@ -522,7 +522,7 @@ cachedPresentData = do
 			put s { presentData = Just v }
 			return v
 
-cachedRemoteData :: UUID -> StatState KeyData
+cachedRemoteData :: UUID -> StatState KeyInfo
 cachedRemoteData u = do
 	s <- get
 	case M.lookup u (repoData s) of
@@ -531,19 +531,19 @@ cachedRemoteData u = do
 			let combinedata d uk = finishCheck uk >>= \case
 				Nothing -> return d
 				Just k -> return $ addKey k d
-			v <- lift $ foldM combinedata emptyKeyData
+			v <- lift $ foldM combinedata emptyKeyInfo
 				=<< loggedKeysFor' u
 			put s { repoData = M.insert u v (repoData s) }
 			return v
 
-cachedReferencedData :: StatState KeyData
+cachedReferencedData :: StatState KeyInfo
 cachedReferencedData = do
 	s <- get
 	case referencedData s of
 		Just v -> return v
 		Nothing -> do
 			!v <- lift $ Command.Unused.withKeysReferenced
-				emptyKeyData addKey
+				emptyKeyInfo addKey
 			put s { referencedData = Just v }
 			return v
 
@@ -552,7 +552,7 @@ cachedNumCopiesStats :: StatState (Maybe NumCopiesStats)
 cachedNumCopiesStats = numCopiesStats <$> get
 
 -- currently only available for directory info
-cachedRepoData :: StatState (M.Map UUID KeyData)
+cachedRepoData :: StatState (M.Map UUID KeyInfo)
 cachedRepoData = repoData <$> get
 
 getDirStatInfo :: InfoOptions -> FilePath -> Annex StatInfo
@@ -564,7 +564,7 @@ getDirStatInfo o dir = do
 			(update matcher fast)
 	return $ StatInfo (Just presentdata) (Just referenceddata) repodata (Just numcopiesstats) o
   where
-	initial = (emptyKeyData, emptyKeyData, emptyNumCopiesStats, M.empty)
+	initial = (emptyKeyInfo, emptyKeyInfo, emptyNumCopiesStats, M.empty)
 	update matcher fast key file vs@(presentdata, referenceddata, numcopiesstats, repodata) =
 		ifM (matcher $ MatchingFile $ FileInfo file file)
 			( do
@@ -594,7 +594,7 @@ getTreeStatInfo o r = do
 		, return Nothing
 		)
   where
-	initial = (emptyKeyData, emptyKeyData, M.empty)
+	initial = (emptyKeyInfo, emptyKeyInfo, M.empty)
 	go _ [] vs = return vs
 	go fast (l:ls) vs@(presentdata, referenceddata, repodata) = do
 		mk <- catKey (LsTree.sha l)
@@ -613,33 +613,33 @@ getTreeStatInfo o r = do
 						return (updateRepoData key locs repodata)
 				go fast ls $! (presentdata', referenceddata', repodata')
 
-emptyKeyData :: KeyData
-emptyKeyData = KeyData 0 0 0 M.empty
+emptyKeyInfo :: KeyInfo
+emptyKeyInfo = KeyInfo 0 0 0 M.empty
 
 emptyNumCopiesStats :: NumCopiesStats
 emptyNumCopiesStats = NumCopiesStats M.empty
 
-foldKeys :: [Key] -> KeyData
-foldKeys = foldl' (flip addKey) emptyKeyData
+foldKeys :: [Key] -> KeyInfo
+foldKeys = foldl' (flip addKey) emptyKeyInfo
 
-addKey :: Key -> KeyData -> KeyData
-addKey key (KeyData count size unknownsize backends) =
-	KeyData count' size' unknownsize' backends'
+addKey :: Key -> KeyInfo -> KeyInfo
+addKey key (KeyInfo count size unknownsize backends) =
+	KeyInfo count' size' unknownsize' backends'
   where
 	{- All calculations strict to avoid thunks when repeatedly
 	 - applied to many keys. -}
 	!count' = count + 1
-	!backends' = M.insertWith (+) (keyVariety key) 1 backends
+	!backends' = M.insertWith (+) (fromKey keyVariety key) 1 backends
 	!size' = maybe size (+ size) ks
 	!unknownsize' = maybe (unknownsize + 1) (const unknownsize) ks
-	ks = keySize key
+	ks = fromKey keySize key
 
-updateRepoData :: Key -> [UUID] -> M.Map UUID KeyData -> M.Map UUID KeyData
+updateRepoData :: Key -> [UUID] -> M.Map UUID KeyInfo -> M.Map UUID KeyInfo
 updateRepoData key locs m = m'
   where
 	!m' = M.unionWith (\_old new -> new) m $
 		M.fromList $ zip locs (map update locs)
-	update loc = addKey key (fromMaybe emptyKeyData $ M.lookup loc m)
+	update loc = addKey key (fromMaybe emptyKeyInfo $ M.lookup loc m)
 
 updateNumCopiesStats :: FilePath -> NumCopiesStats -> [UUID] -> Annex NumCopiesStats
 updateNumCopiesStats file (NumCopiesStats m) locs = do
@@ -649,7 +649,7 @@ updateNumCopiesStats file (NumCopiesStats m) locs = do
 	let !ret = NumCopiesStats m'
 	return ret
 
-showSizeKeys :: KeyData -> StatState String
+showSizeKeys :: KeyInfo -> StatState String
 showSizeKeys d = do
 	sizer <- mkSizer
 	return $ total sizer ++ missingnote