Rename ./cargo/config to .cargo/config.toml

This avoids the following warning:
```
warning: `/Users/nicholas/git/slog.org/slog/.cargo/config` is deprecated in favor of `config.toml`
note: if you need to support cargo 1.38 or earlier, you can symlink `config` to `config.toml`
```

Since our MSRV is currently 1.49, we can safely make this change
without needing a system link.

Author: Techcable