Update to protolude 0.3.0

The good part is that protolude 0.3.0 builds with GHC 8.10.

The bad part is that this change is a bit painful:
- the default `toS` has changed to no longer convert to and from ByteString
- similarly, `show` no longer outputs ByteString

The changes here are pretty much minimal to keep things compiling; I didn't
see a nice way to work with the new ConvertText class, even though `toUtf8`
seems like it might help if used besides `toS` at just the right spots.

test/SpecHelper.hs

@@ -24,7 +24,8 @@ import Text.Heredoc
 
 import PostgREST.Config (AppConfig (..))
 import PostgREST.Types  (JSPathExp (..))
-import Protolude
+import Protolude        hiding (toS)
+import Protolude.Conv   (toS)
 
 matchContentTypeJson :: MatchHeader
 matchContentTypeJson = "Content-Type" <:> "application/json; charset=utf-8"