hnix-store-remote: implement buildPaths op

I was hoping to use the `StorePath` type from `System.Nix.StorePath`,
but I am not sure how to construct its values at runtime. I ended up
giving `buildPaths` a simpler type that expects a list of bytestrings.

hnix-store-remote/src/System/Nix/Store/Remote.hs

@@ -6,14 +6,17 @@
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications    #-}
 module System.Nix.Store.Remote (
-    runStore
+    BuildMode(..)
+  , runStore
   , syncWithGC
   , optimiseStore
   , verifyStore
+  , buildPaths
   ) where
 
-import           Control.Monad
-
+import           Data.Binary.Put (Put, putInthost)
+import           Data.ByteString (ByteString)
+import           System.Nix.Util
 import           System.Nix.Store.Remote.Types
 import           System.Nix.Store.Remote.Protocol
 import           System.Nix.Store.Remote.Util
@@ -31,3 +34,24 @@ verifyStore :: CheckFlag -> RepairFlag -> MonadStore ()
 verifyStore check repair = runOpArgs_ VerifyStore $ do
   putBool check
   putBool repair
+
+data BuildMode = Normal | Repair | Check
+  deriving (Eq, Show)
+
+putBuildMode :: BuildMode -> Put
+putBuildMode mode = putInthost $
+  case mode of
+    Normal -> 0
+    Repair -> 1
+    Check -> 2
+
+buildPaths ::
+  -- forall storeDir . (KnownStoreDir storeDir) =>
+  -- [StorePath storeDir]
+  [ByteString] -> BuildMode -> MonadStore ()
+buildPaths drvs mode =
+  runOpArgs_ BuildPaths args
+  where
+    args = do
+      putByteStrings drvs
+      putBuildMode mode