CIP Proposal - Open the 'A' Namespace

This thread is about a potential CIP that would allow for non-numeric ‘A’ asset names.

I have some code that I think covers most of it here, for review and feedback:

Non-numeric asset names starting with ‘A’ are valid.
Check that assets are numeric rather than “Starts with ‘A’”

Testnet tx fixture for parsing.

The open question I have, for more experienced devs, is whether this kind of change requires a block index check and where would that go?