Reorganize the repo structure (#2503)

Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

contracts/token/ERC721/utils/ERC721Holder.sol

@@ -0,0 +1,23 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../IERC721Receiver.sol";
+
+  /**
+   * @dev Implementation of the {IERC721Receiver} interface.
+   *
+   * Accepts all token transfers.
+   * Make sure the contract is able to use its token with {IERC721-safeTransferFrom}, {IERC721-approve} or {IERC721-setApprovalForAll}.
+   */
+contract ERC721Holder is IERC721Receiver {
+
+    /**
+     * @dev See {IERC721Receiver-onERC721Received}.
+     *
+     * Always returns `IERC721Receiver.onERC721Received.selector`.
+     */
+    function onERC721Received(address, address, uint256, bytes memory) public virtual override returns (bytes4) {
+        return this.onERC721Received.selector;
+    }
+}