Apply recommendations from hlint

What?
=====

This updates the codebase to switch from data to newtype in certain
cases.

src/Unused/Cache/DirectoryFingerprint.hs

@@ -17,7 +17,7 @@ newtype MD5ExecutablePath = MD5ExecutablePath
 
 type MD5Config = ReaderT MD5ExecutablePath IO
 
-data FingerprintOutcome =
+newtype FingerprintOutcome =
     MD5ExecutableNotFound [String]
 
 sha :: IO (Either FingerprintOutcome String)

src/Unused/Types.hs

@@ -70,11 +70,11 @@ data Removal = Removal
     , rReason :: String
     } deriving (Eq, Show)
 
-data GitContext = GitContext
+newtype GitContext = GitContext
     { gcCommits :: [GitCommit]
     } deriving (Eq, Show)
 
-data GitCommit = GitCommit
+newtype GitCommit = GitCommit
     { gcSha :: String
     } deriving (Eq, Show)