feature request: Add `set_code()` API method

* Version: 0.7.0b1
* Python: 3.9
* OS: macOS

### What was wrong?

Looking for adding more options for `set_code()` to our framework and so this request is to have a way to change the contract code using `eth-tester`.

### How can it be fixed?

```
def set_code(self, address: AddressType, code: ContractCode) -> bool:
vm = _get_vm_for_block_number(self.evm_backend.chain, "latest")
vm.state.set_code(HexBytes(address), code)
return True
```