Making IERC721Receiver an interface (#2225)

* Making IERC721Receiver an interface

* Update IERC721Receiver.sol

Co-authored-by: Nicolás Venturo <nicolas.venturo@gmail.com>

contracts/token/ERC721/IERC721Receiver.sol

@@ -5,7 +5,7 @@ pragma solidity ^0.6.0;
  * @dev Interface for any contract that wants to support safeTransfers
  * from ERC721 asset contracts.
  */
-abstract contract IERC721Receiver {
+interface IERC721Receiver {
     /**
      * @notice Handle the receipt of an NFT
      * @dev The ERC721 smart contract calls this function on the recipient
@@ -20,6 +20,6 @@ abstract contract IERC721Receiver {
      * @param data Additional data with no specified format
      * @return bytes4 `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`
      */
-    function onERC721Received(address operator, address from, uint256 tokenId, bytes memory data)
-    public virtual returns (bytes4);
+    function onERC721Received(address operator, address from, uint256 tokenId, bytes calldata data)
+    external returns (bytes4);
 }