Add ERC20 contract (mostly for testing)

Subtask of #6 

For building and running tests of transaction with fee currencies, we need an ERC20 implementation. 

```[tasklist]
### Tasks
- [x] Find a suitable token with `credit` and `debit` functions
- [x] Add it to the https://github.com/celo-org/optimism repo
- [x] Adapt the import script and generate wrappers: https://github.com/celo-org/op-geth/tree/celo1/contracts/celo
```
