Rearrange erc721metadata layout to mimic eth-pkg (#2098)

In https://github.com/OpenZeppelin/openzeppelin-contracts-ethereum-package/pull/76, we are rearranging the erc721metadata contract storage layout, so the new variable is added at the end. This commit applies the same change to the vanilla contracts repository, so migration from 2.5 to 3.0 is easier for users using the transpiler.

contracts/token/ERC721/ERC721Metadata.sol

@@ -12,12 +12,12 @@ contract ERC721Metadata is Context, ERC165, ERC721, IERC721Metadata {
     // Token symbol
     string private _symbol;
 
-    // Base URI
-    string private _baseURI;
-
     // Optional mapping for token URIs
     mapping(uint256 => string) private _tokenURIs;
 
+    // Base URI
+    string private _baseURI;
+
     /*
      *     bytes4(keccak256('name()')) == 0x06fdde03
      *     bytes4(keccak256('symbol()')) == 0x95d89b41