Simplify ERC20 extension

[martriay]

I was thinking that maybe we could abstract ERC20 even further and move transferFrom, increaseAllowance, etc. implementations to the base contract, and require frontend contracts just to expose them instead of re-implementing them. What do you think?

[andrew-fleming]

I was considering creating a second contract for frontend contracts from which to import the external methods since we're rewriting them per instance; however, it would be easier and better if we just held them in the base contract. I only see upside here