add ExportTree table to export db

New table needed to look up what filenames are used in the currently
exported tree, for reasons explained in export.mdwn.

Also, added smart constructors for ExportLocation and ExportDirectory to
make sure they contain filepaths with the right direction slashes.

And some code refactoring.

This commit was sponsored by Francois Marier on Patreon.

Remote/Directory.hs

@@ -274,14 +274,14 @@ renameExportM d _k oldloc newloc = liftIO $ catchBoolIO $ do
 	dest = exportPath d newloc
 
 exportPath :: FilePath -> ExportLocation -> FilePath
-exportPath d (ExportLocation loc) = d </> loc
+exportPath d loc = d </> fromExportLocation loc
 
 {- Removes the ExportLocation directory and its parents, so long as
  - they're empty, up to but not including the topdir. -}
 removeExportLocation :: FilePath -> ExportLocation -> IO ()
-removeExportLocation topdir (ExportLocation loc) = go (Just loc) (Right ())
+removeExportLocation topdir loc = go (Just $ fromExportLocation loc) (Right ())
   where
 	go _ (Left _e) = return ()
 	go Nothing _ = return ()
 	go (Just loc') _ = go (upFrom loc')
-		=<< tryIO (removeDirectory $ exportPath topdir (ExportLocation loc'))
+		=<< tryIO (removeDirectory $ exportPath topdir (mkExportLocation loc'))