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

python-packages/order_utils/test/test_doctest.py

@@ -1,10 +1,24 @@
 """Exercise doctests for order_utils module."""
 
 from doctest import testmod
-from zero_ex.order_utils import signature_utils
+
+from zero_ex.dev_utils import abi_utils, type_assertions
+from zero_ex.order_utils import asset_data_utils
 
 
-def test_doctest():
-    """Invoke doctest on the module."""
-    (failure_count, _) = testmod(signature_utils)
+def test_doctest_asset_data_utils():
+    """Invoke doctest on the asset_data_utils module."""
+    (failure_count, _) = testmod(asset_data_utils)
+    assert failure_count == 0
+
+
+def test_doctest_abi_utils():
+    """Invoke doctest on the abi_utils module."""
+    (failure_count, _) = testmod(abi_utils)
+    assert failure_count == 0
+
+
+def test_doctest_type_assertions():
+    """Invoke doctest on the type_assertions module."""
+    (failure_count, _) = testmod(type_assertions)
     assert failure_count == 0