fix corpus path for loading and saving (#919)

lib/Echidna/Output/Corpus.hs

@@ -8,13 +8,14 @@ import Data.ByteString qualified as BS
 import Data.Hashable (hash)
 import Data.Maybe (catMaybes)
 import System.Directory (createDirectoryIfMissing, makeRelativeToCurrentDirectory, doesFileExist)
+import System.FilePath ((</>), (<.>))
 
 import Echidna.Types.Tx
 import Echidna.Output.Utils
 
 saveTxs :: FilePath -> [[Tx]] -> IO ()
 saveTxs d = mapM_ saveTx where
-  saveTx v = do let fn = d ++ (show . hash . show) v ++ ".txt"
+  saveTx v = do let fn = d </> (show . hash . show) v <.> "txt"
                 b <- doesFileExist fn
                 unless b $ encodeFile fn (toJSON v)