feat: Add sequential swap methods

- Add some basic tests (will add more in next commits)
- Adapt sequential tests (which originally tested split swap)
- Adapt a forgotten single swap test

TODO:
- Fix encoding of single and sequential swaps to not expect the split sawp format every time (the split and the token indices are not necessary and consume unnecessary gas).

foundry/test/TychoRouterTestSetup.sol

@@ -29,6 +29,13 @@ contract TychoRouterExposed is TychoRouter {
     ) external returns (uint256) {
         return _splitSwap(amountIn, nTokens, swaps);
     }
+
+    function exposedSequentialSwap(
+        uint256 amountIn,
+        bytes calldata swaps
+    ) external returns (uint256) {
+        return _sequentialSwap(amountIn, swaps);
+    }
 }
 
 contract TychoRouterTestSetup is Constants {
@@ -193,6 +200,18 @@ contract TychoRouterTestSetup is Constants {
         );
     }
 
+    function encodeSequentialSwap(
+        uint8 tokenInIndex,
+        uint8 tokenOutIndex,
+        uint24 split,
+        address executor,
+        bytes memory protocolData
+    ) internal pure returns (bytes memory) {
+        return abi.encodePacked(
+            tokenInIndex, tokenOutIndex, split, executor, protocolData
+        );
+    }
+
     function encodeUniswapV2Swap(
         address tokenIn,
         address target,