switch MetaValue to ByteString and MetaField to Text

MetaField was already limited to alphanumerics, so it makes sense to use
Text for it.

Note that technically a UUID can contain invalid UTF-8, and so
remoteMetaDataPrefix's use of T.pack . fromUUID could replace non-UTF8
values with '?' or whatever. In practice, a UUID is usually also text,
I only kept open the possibility of it containing invalid UTF-8 to avoid
breaking parsing of strange UUIDs in git-annex branch files. So, I
decided to let this edge case slip by.

Have not updated the rest of the code base yet for this change, as the
change took 2.5 hours longer than I expected to get working properly.

Utility/Base64.hs

@@ -1,39 +1,49 @@
-{- Simple Base64 encoding of Strings
+{- Simple Base64 encoding
  -
- - Note that this uses the FileSystemEncoding, so it can be used on Strings
- - that repesent filepaths containing arbitrarily encoded characters.
- -
- - Copyright 2011, 2015 Joey Hess <id@joeyh.name>
+ - Copyright 2011-2019 Joey Hess <id@joeyh.name>
  -
  - License: BSD-2-clause
  -}
 
-module Utility.Base64 (toB64, fromB64Maybe, fromB64, prop_b64_roundtrips) where
+module Utility.Base64 where
 
 import Utility.FileSystemEncoding
 
 import qualified "sandi" Codec.Binary.Base64 as B64
 import Data.Maybe
-import qualified Data.ByteString.Lazy as L
+import qualified Data.ByteString as B
 import Data.ByteString.UTF8 (fromString, toString)
 import Data.Char
 
-toB64 :: String -> String	
+-- | This uses the FileSystemEncoding, so it can be used on Strings
+-- that repesent filepaths containing arbitrarily encoded characters.
+toB64 :: String -> String
 toB64 = toString . B64.encode . encodeBS
 
+toB64' :: B.ByteString -> B.ByteString
+toB64' = B64.encode
+
 fromB64Maybe :: String -> Maybe String
-fromB64Maybe s = either (const Nothing) (Just . decodeBL . L.fromStrict)
+fromB64Maybe s = either (const Nothing) (Just . decodeBS)
 	(B64.decode $ fromString s)
 
+fromB64Maybe' :: B.ByteString -> Maybe (B.ByteString)
+fromB64Maybe' = either (const Nothing) Just . B64.decode
+
 fromB64 :: String -> String
 fromB64 = fromMaybe bad . fromB64Maybe
   where
 	bad = error "bad base64 encoded data"
 
+fromB64' :: B.ByteString -> B.ByteString
+fromB64' = fromMaybe bad . fromB64Maybe'
+  where
+	bad = error "bad base64 encoded data"
+
 -- Only ascii strings are tested, because an arbitrary string may contain
 -- characters not encoded using the FileSystemEncoding, which would thus
--- not roundtrip, as fromB64 always generates an output encoded that way.
+-- not roundtrip, as decodeBS always generates an output encoded that way.
 prop_b64_roundtrips :: String -> Bool
 prop_b64_roundtrips s
-	| all (isAscii) s = s == fromB64 (toB64 s)
+	| all (isAscii) s = s == decodeBS (fromB64' (toB64' (encodeBS s)))
 	| otherwise = True