feat(order_utils.py): ERC20 asset data encoding and decoding

In addition to the ERC20 codec, also:

Stopped ignoring type errors on 3rd party imports, by including
interface stubs for them;
Removed the unimplemented signature-utils module, which was just a
stand-in when the python project support was first put in place.

https://github.com/0xProject/0x-monorepo/pull/1144

packages/order-utils/test/asset_data_utils_test.ts

@@ -0,0 +1,31 @@
+import * as chai from 'chai';
+
+import { ERC20AssetData } from '@0x/types';
+
+import { assetDataUtils } from '../src/asset_data_utils';
+
+import { chaiSetup } from './utils/chai_setup';
+
+chaiSetup.configure();
+const expect = chai.expect;
+
+const KNOWN_ENCODINGS = [
+    {
+        address: '0x1dc4c1cefef38a777b15aa20260a54e584b16c48',
+        assetData: '0xf47261b00000000000000000000000001dc4c1cefef38a777b15aa20260a54e584b16c48',
+    },
+];
+
+const ERC20_ASSET_PROXY_ID = '0xf47261b0';
+
+describe('assetDataUtils', () => {
+    it('should encode', () => {
+        const assetData = assetDataUtils.encodeERC20AssetData(KNOWN_ENCODINGS[0].address);
+        expect(assetData).to.equal(KNOWN_ENCODINGS[0].assetData);
+    });
+    it('should decode', () => {
+        const assetData: ERC20AssetData = assetDataUtils.decodeERC20AssetData(KNOWN_ENCODINGS[0].assetData);
+        expect(assetData.tokenAddress).to.equal(KNOWN_ENCODINGS[0].address);
+        expect(assetData.assetProxyId).to.equal(ERC20_ASSET_PROXY_ID);
+    });
+});