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/stubs/setuptools/command/test.pyi

@@ -0,0 +1,3 @@
+from setuptools import Command
+
+class test(Command): ...