Consider add permissionless mint and burn for MockERC20

[robinsdan]

This contract was proposed in #469, and the mint/burn function was not included because the balance can be set through "deal". At the same time, it can be easily added if needed.

However, the absence of mint/burn poses two inconveniences:

1. "deal" can only be used for local testing, which means that when the contract needs to be deployed to the testnet, a new contract with "mint" needs to be re-implemented.
2. Using "deal" results in a significant number of additional calls, making the call trace very cumbersome.