convert Key to ShortByteString

This adds the overhead of a copy when serializing and deserializing keys.
I have not benchmarked much, but runtimes seem barely changed at all by that.

When a lot of keys are in memory, it improves memory use.

And, it prevents keys sometimes getting PINNED in memory and failing to GC,
which is a problem ByteString has sometimes. In particular, git-annex sync
from a borg special remote had that problem and this improved its memory
use by a large amount.

Sponsored-by: Shae Erisson on Patreon

Backend.hs

@@ -33,6 +33,7 @@ import qualified Backend.URL
 import qualified Backend.External
 
 import qualified Data.Map as M
+import qualified Data.ByteString.Short as S (toShort, fromShort)
 import qualified Data.ByteString.Char8 as S8
 
 {- Build-in backends. Does not include externals. -}
@@ -67,7 +68,7 @@ genKey source meterupdate preferredbackend = do
   where
 	-- keyNames should not contain newline characters.
 	makesane k = alterKey k $ \d -> d
-		{ keyName = S8.map fixbadchar (fromKey keyName k)
+		{ keyName = S.toShort (S8.map fixbadchar (S.fromShort (fromKey keyName k)))
 		}
 	fixbadchar c
 		| c == '\n' = '_'