Added
- `Provider.readonly` constructor for using AnchorPy only to fetch data
[(58)](https://github.com/kevinheavey/anchorpy/pull/58)
- `commitment` parameter in `.fetch` and `.fetch_multiple` methods
[(58)](https://github.com/kevinheavey/anchorpy/pull/58)
Fixed
- Cache some generated Python types to avoid issues with checking equality
[(57)](https://github.com/kevinheavey/anchorpy/pull/57)