Things like `luma(1, key: "val")` didn't produce an error before because `args.finish()?` wasn't called. This changes `args: Args` to `args: &mut Args` to make it impossible for that to happen.
- Moves `Bytes` from `util` to `eval` module
- Accepts bytes in `str` function for bytes -> str conversion
- Adds `bytes` function for str | array -> bytes conversion
- Adds `array` function for bytes -> array conversion
- Adds `len`, `at`, and `slice` methods for bytes
- Adds `encoding` parameter to `read` function