core implementation of new style chunking

Not yet used by any special remotes, but should not be too hard to add it
to most of them.

storeChunks is the hairy bit! It's loosely based on
Remote.Directory.storeLegacyChunked. The object is read in using a lazy
bytestring, which is streamed though, creating chunks as needed, without
ever buffering more than 1 chunk in memory.

Getting the progress meter update to work right was also fun, since
progress meter values are absolute. Finessed by constructing an offset
meter.

This commit was sponsored by Richard Collins.

Utility/Metered.hs

@@ -16,6 +16,7 @@ import qualified Data.ByteString as S
 import System.IO.Unsafe
 import Foreign.Storable (Storable(sizeOf))
 import System.Posix.Types
+import Data.Int
 
 {- An action that can be run repeatedly, updating it on the bytes processed.
  -
@@ -31,6 +32,10 @@ class AsBytesProcessed a where
 	toBytesProcessed :: a -> BytesProcessed
 	fromBytesProcessed :: BytesProcessed -> a
 
+instance AsBytesProcessed BytesProcessed where
+	toBytesProcessed = id
+	fromBytesProcessed = id
+
 instance AsBytesProcessed Integer where
 	toBytesProcessed i = BytesProcessed i
 	fromBytesProcessed (BytesProcessed i) = i
@@ -39,6 +44,10 @@ instance AsBytesProcessed Int where
 	toBytesProcessed i = BytesProcessed $ toInteger i
 	fromBytesProcessed (BytesProcessed i) = fromInteger i
 
+instance AsBytesProcessed Int64 where
+	toBytesProcessed i = BytesProcessed $ toInteger i
+	fromBytesProcessed (BytesProcessed i) = fromInteger i
+
 instance AsBytesProcessed FileOffset where
 	toBytesProcessed sz = BytesProcessed $ toInteger sz
 	fromBytesProcessed (BytesProcessed sz) = fromInteger sz
@@ -77,6 +86,13 @@ meteredWriteFile :: MeterUpdate -> FilePath -> L.ByteString -> IO ()
 meteredWriteFile meterupdate f b = withBinaryFile f WriteMode $ \h ->
 	meteredWrite meterupdate h b
 
+{- Applies an offset to a MeterUpdate. This can be useful when
+ - performing a sequence of actions, such as multiple meteredWriteFiles,
+ - that all update a common meter progressively.
+ -}
+offsetMeterUpdate :: MeterUpdate -> BytesProcessed -> MeterUpdate
+offsetMeterUpdate base offset = \n -> base (offset `addBytesProcessed` n)
+
 {- This is like L.hGetContents, but after each chunk is read, a meter
  - is updated based on the size of the chunk.
  -