Improved handling of negative constants (#683)

lib/Echidna/ABI.hs

@@ -59,7 +59,7 @@ mkValidAbiUInt :: Int -> Word256 -> Maybe AbiValue
 mkValidAbiUInt i x = if x <= 2 ^ i - 1 then Just $ AbiUInt i x else Nothing
 
 makeNumAbiValues :: Integer -> [AbiValue]
-makeNumAbiValues i = let l f = f <$> commonTypeSizes <*> fmap fromIntegral [i-1..i+1] in
+makeNumAbiValues i = let l f = f <$> commonTypeSizes <*> fmap fromIntegral ([i-1..i+1] ++ [(-i)-1 .. (-i)+1]) in
     catMaybes (l mkValidAbiInt ++ l mkValidAbiUInt)
 
 makeArrayAbiValues :: BS.ByteString -> [AbiValue]