improve data types for sqlite

This is a non-backwards compatable change, so not suitable for merging
w/o a annex.version bump and transition code. Not yet tested.

This improves performance of git-annex benchmark --databases
across the board by 10-25%, since eg Key roundtrips as a ByteString.

(serializeKey' produces a lazy ByteString, so there is still a
copy involved in converting it to a strict ByteString. It may be faster
to switch to using bytestring-strict-builder.)

FilePath and Key are both stored as blobs. This avoids mojibake in some
situations. It would be possible to use varchar instead, if persistent
could avoid converting that to Text, but it seems there is no good
way to do so. See doc/todo/sqlite_database_improvements.mdwn

Eliminated some ugly artifacts of using Read/Show serialization;
constructors and quoted strings are no longer stored in sqlite.

Renamed SRef to SSha to reflect that it is only ever a git sha,
not a ref name. Since it is limited to the characters in a sha,
it is not affected by mojibake, so still uses String.

Key.hs

@@ -140,7 +140,7 @@ deserializeKey :: String -> Maybe Key
 deserializeKey = deserializeKey' . encodeBS'
 
 deserializeKey' :: S.ByteString -> Maybe Key
-deserializeKey' b = eitherToMaybe $ A.parseOnly keyParser b
+deserializeKey' = eitherToMaybe . A.parseOnly keyParser
 
 {- This splits any extension out of the keyName, returning the 
  - keyName minus extension, and the extension (including leading dot).