add KeyVariety type

Where before the "name" of a key and a backend was a string, this makes
it a concrete data type.

This is groundwork for allowing some varieties of keys to be disabled
in file2key, so git-annex won't use them at all.

Benchmarks ran in my big repo:

old git-annex info:

real	0m3.338s
user	0m3.124s
sys	0m0.244s

new git-annex info:

real	0m3.216s
user	0m3.024s
sys	0m0.220s

new git-annex find:

real	0m7.138s
user	0m6.924s
sys	0m0.252s

old git-annex find:

real	0m7.433s
user	0m7.240s
sys	0m0.232s

Surprising result; I'd have expected it to be slower since it now parses
all the key varieties. But, the parser is very simple and perhaps
sharing KeyVarieties uses less memory or something like that.

This commit was supported by the NSF-funded DataLad project.

Command/Info.hs

@@ -36,6 +36,7 @@ import qualified Git.LsTree as LsTree
 import Utility.Percentage
 import Types.Transfer
 import Logs.Transfer
+import Types.Key
 import Types.TrustLevel
 import Types.FileMatcher
 import qualified Limit
@@ -51,7 +52,7 @@ data KeyData = KeyData
 	{ countKeys :: Integer
 	, sizeKeys :: Integer
 	, unknownSizeKeys :: Integer
-	, backendsKeys :: M.Map String Integer
+	, backendsKeys :: M.Map KeyVariety Integer
 	}
 
 data NumCopiesStats = NumCopiesStats
@@ -451,7 +452,8 @@ disk_size = simpleStat "available local disk space" $
 
 backend_usage :: Stat
 backend_usage = stat "backend usage" $ json fmt $
-	ObjectMap . backendsKeys <$> cachedReferencedData
+	ObjectMap . (M.mapKeys formatKeyVariety) . backendsKeys
+		<$> cachedReferencedData
   where
 	fmt = multiLine . map (\(b, n) -> b ++ ": " ++ show n) . sort . M.toList . fromObjectMap
 
@@ -598,7 +600,7 @@ addKey key (KeyData count size unknownsize backends) =
 	{- All calculations strict to avoid thunks when repeatedly
 	 - applied to many keys. -}
 	!count' = count + 1
-	!backends' = M.insertWith (+) (keyBackendName key) 1 backends
+	!backends' = M.insertWith (+) (keyVariety key) 1 backends
 	!size' = maybe size (+ size) ks
 	!unknownsize' = maybe (unknownsize + 1) (const unknownsize) ks
 	ks = keySize key