- Refactor assetWrapper to contain more of the normalizing logic instead of erc20Wrapper and erc721Wrapper

- Add burn method to DummyERC721Token
- Add additional methods to assetWrapper to set balance/allowances on ERC20 and ERC721 tokens
- Use approve instead of approveAll for ERC721 tokens

packages/contracts/test/exchange/wrapper.ts

@@ -369,7 +369,7 @@ describe('Exchange wrappers', () => {
                 // HACK(albrow): We need to hardcode the gas estimate here because
                 // the Geth gas estimator doesn't work with the way we use
                 // delegatecall and swallow errors.
-                gas: 270000,
+                gas: 280000,
             });
             // Verify post-conditions
             const newOwnerMakerAsset = await erc721Token.ownerOf.callAsync(makerAssetId);