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.

Limit.hs

@@ -294,7 +294,7 @@ addInBackend = addLimit . limitInBackend
 limitInBackend :: MkLimit Annex
 limitInBackend name = Right $ const $ checkKey check
   where
-	check key = pure $ keyVariety key == variety
+	check key = pure $ fromKey keyVariety key == variety
 	variety = parseKeyVariety (encodeBS name)
 
 {- Adds a limit to skip files not using a secure hash. -}
@@ -302,7 +302,7 @@ addSecureHash :: Annex ()
 addSecureHash = addLimit $ Right limitSecureHash
 
 limitSecureHash :: MatchFiles Annex
-limitSecureHash _ = checkKey $ pure . cryptographicallySecure . keyVariety
+limitSecureHash _ = checkKey $ pure . cryptographicallySecure . fromKey keyVariety
 
 {- Adds a limit to skip files that are too large or too small -}
 addLargerThan :: String -> Annex ()
@@ -327,7 +327,7 @@ limitSize lb vs s = case readSize dataUnits s of
 	go sz _ (MatchingInfo p) =
 		getInfo (providedFileSize p) 
 			>>= \sz' -> return (Just sz' `vs` Just sz)
-	checkkey sz key = return $ keySize key `vs` Just sz
+	checkkey sz key = return $ fromKey keySize key `vs` Just sz
 
 addMetaData :: String -> Annex ()
 addMetaData = addLimit . limitMetaData