Make IERC721 contracts interfaces instead (#2113)

* Make IERC721 contracts interfaces instead

* Bump minimum compiler version for IERC721

contracts/token/ERC721/IERC721.sol

-pragma solidity ^0.6.0;
+pragma solidity ^0.6.2;
 
 import "../../introspection/IERC165.sol";
 
 /**
  * @dev Required interface of an ERC721 compliant contract.
  */
-abstract contract IERC721 is IERC165 {
+interface IERC721 is IERC165 {
     event Transfer(address indexed from, address indexed to, uint256 indexed tokenId);
     event Approval(address indexed owner, address indexed approved, uint256 indexed tokenId);
     event ApprovalForAll(address indexed owner, address indexed operator, bool approved);
@@ -13,12 +13,12 @@ abstract contract IERC721 is IERC165 {
     /**
      * @dev Returns the number of NFTs in `owner`'s account.
      */
-    function balanceOf(address owner) public view virtual returns (uint256 balance);
+    function balanceOf(address owner) external view returns (uint256 balance);
 
     /**
      * @dev Returns the owner of the NFT specified by `tokenId`.
      */
-    function ownerOf(uint256 tokenId) public view virtual returns (address owner);
+    function ownerOf(uint256 tokenId) external view returns (address owner);
 
     /**
      * @dev Transfers a specific NFT (`tokenId`) from one account (`from`) to
@@ -32,7 +32,7 @@ abstract contract IERC721 is IERC165 {
      * - If the caller is not `from`, it must be have been allowed to move this
      * NFT by either {approve} or {setApprovalForAll}.
      */
-    function safeTransferFrom(address from, address to, uint256 tokenId) public virtual;
+    function safeTransferFrom(address from, address to, uint256 tokenId) external;
     /**
      * @dev Transfers a specific NFT (`tokenId`) from one account (`from`) to
      * another (`to`).
@@ -41,13 +41,13 @@ abstract contract IERC721 is IERC165 {
      * - If the caller is not `from`, it must be approved to move this NFT by
      * either {approve} or {setApprovalForAll}.
      */
-    function transferFrom(address from, address to, uint256 tokenId) public virtual;
-    function approve(address to, uint256 tokenId) public virtual;
-    function getApproved(uint256 tokenId) public view virtual returns (address operator);
+    function transferFrom(address from, address to, uint256 tokenId) external;
+    function approve(address to, uint256 tokenId) external;
+    function getApproved(uint256 tokenId) external view returns (address operator);
 
-    function setApprovalForAll(address operator, bool _approved) public virtual;
-    function isApprovedForAll(address owner, address operator) public view virtual returns (bool);
+    function setApprovalForAll(address operator, bool _approved) external;
+    function isApprovedForAll(address owner, address operator) external view returns (bool);
 
 
-    function safeTransferFrom(address from, address to, uint256 tokenId, bytes memory data) public virtual;
+    function safeTransferFrom(address from, address to, uint256 tokenId, bytes calldata data) external;
 }

contracts/token/ERC721/IERC721Enumerable.sol

-pragma solidity ^0.6.0;
+pragma solidity ^0.6.2;
 
 import "./IERC721.sol";
 
@@ -6,9 +6,9 @@ import "./IERC721.sol";
  * @title ERC-721 Non-Fungible Token Standard, optional enumeration extension
  * @dev See https://eips.ethereum.org/EIPS/eip-721
  */
-abstract contract IERC721Enumerable is IERC721 {
-    function totalSupply() public view virtual returns (uint256);
-    function tokenOfOwnerByIndex(address owner, uint256 index) public view virtual returns (uint256 tokenId);
+interface IERC721Enumerable is IERC721 {
+    function totalSupply() external view returns (uint256);
+    function tokenOfOwnerByIndex(address owner, uint256 index) external view returns (uint256 tokenId);
 
-    function tokenByIndex(uint256 index) public view virtual returns (uint256);
+    function tokenByIndex(uint256 index) external view returns (uint256);
 }

contracts/token/ERC721/IERC721Metadata.sol

-pragma solidity ^0.6.0;
+pragma solidity ^0.6.2;
 
 import "./IERC721.sol";
 
@@ -6,8 +6,8 @@ import "./IERC721.sol";
  * @title ERC-721 Non-Fungible Token Standard, optional metadata extension
  * @dev See https://eips.ethereum.org/EIPS/eip-721
  */
-abstract contract IERC721Metadata is IERC721 {
-    function name() external view virtual returns (string memory);
-    function symbol() external view virtual returns (string memory);
-    function tokenURI(uint256 tokenId) external view virtual returns (string memory);
+interface IERC721Metadata is IERC721 {
+    function name() external view returns (string memory);
+    function symbol() external view returns (string memory);
+    function tokenURI(uint256 tokenId) external view returns (string memory);
 }