Refactor cw20-base to use controller pattern

[ethanfrey]

This should be an easier and more extensible way to extend it than the current function dispatching.

So we can call the handle_X and query_X functions, or the lower-level functionality of the controller if we need to customize more.

• Allowances
• Balances

When done, update

• cw20-staking
• cw20-bonding

to make use of them. This should be easier to override and customize. Note the "sub dispatches" to handle_burn, which is a kind of ugly. It is nice to have a slightly lower-level function exposed to be imported by other handle functions if they want to customize.

[ethanfrey]

This is an old idea, made obsolete by #391 which is much more developed idea along similar lines