Update erc20 contract to 0.8

[webmaster128]

Based on #67

Part of #65

[ethanfrey]

Looks good.

But if you are open, I can suggest a much more radical change using Uint128 for input types and making full use of cosmwasm storage. If you look at the staking demo contract, we have transfer and query balance implemented in many fewer lines. I don't know if it is too much magic for you, but I think this would make this contract much easier to extend.

[ethanfrey]

We were discussing to remove the human / canonical address division if we use a binary encoding scheme, so we can just pass in canonical addresses.

This is one place where we will want to convert to human readable in any case. So we will need the hooks, just have to use them much less

[webmaster128]

But if you are open, I can suggest a much more radical change using Uint128 for input types and making full use of cosmwasm storage.

Updated the public interfaces to Uint128 – good point. One helper function less we need. However, I kept the balance and allowance storage 16 raw bytes, which is more compact that using JSON encoded decimal strings. We can change that later if we want.

[ethanfrey]

Fair enough, let's merge this!