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_signature_utils.py

@@ -1,8 +0,0 @@
-"""Tests of 0x.order_utils.signature_utils.*."""
-
-from zero_ex.order_utils.signature_utils import ec_sign_order_hash
-
-
-def test_ec_sign_order_hash():
-    """Test the signing of order hashes."""
-    assert ec_sign_order_hash() == "stub return value"