Support PartialEq on error

[ethanfrey]

Note: this is a long background process. Let's update contracts one by one as we refactor them (a bit like updating them all to storage-plus)

With StdError::PartialEq implementation from CosmWasm/cosmwasm#662, we can replace lots of statements like:

        match err {
            ContractError::Unauthorized {} => {}
            e => panic!("Unexpected error: {}", e),
        }

with simpler statements like:

assert_eq!(ContractError::Unauthorized {}, err);

It requires #[derive(PartialEq)] on ContractError and the above-mentioned support for StdError

Contracts to update:

• cw1-subkeys
• cw1-whitelist
• cw3-fixed-multisig
• cw3-flex-multisig
• cw4-group
• cw4-stake
• cw20-atomic-swap
• cw20-base
• cw20-bonding
• cw20-escrow
• cw20-staking
• cw721-base

[ethanfrey]

This works now, and we just need to update the contract code to use it.
I will add a bunch of checkboxes and tick them off as I port the contract code (I did a few during other refactorings)

[maurolacy]

Are you still working on this, or should I take it from here?

Update: I'll start working from top to bottom items, in the list above. And, checking the item when starting.

[ethanfrey]

Thanks for taking this over. I started but then left it. Your PR looks nice.