add back debug logging

Make Utility.Process wrap the parts of System.Process that I use,
and add debug logging to them.

Also wrote some higher-level code that allows running an action
with handles to a processes stdin or stdout (or both), and checking
its exit status, all in a single function call.

As a bonus, the debug logging now indicates whether the process
is being run to read from it, feed it data, chat with it (writing and
reading), or just call it for its side effect.

Backend/SHA.hs

@@ -12,7 +12,6 @@ import qualified Annex
 import Types.Backend
 import Types.Key
 import Types.KeySource
-import System.Process
 
 import qualified Build.SysConfig as SysConfig
 import Data.Digest.Pure.SHA