Better configuration ergonomics

I find setting a desired configuration rather tedious. The following things would help:

- Make all constructors `const` so that they may be used in constants
- Provide some constants for common use cases. I suggest having one for the most compact configuration (I think this is just `DefaultConfig`) and one that is fully self-describing
- If breaking backwards-compatibility is an option, I'd design the structs differently: Maybe have one struct for the configuration, with one generic parameter for each option: `Config` or something. Maybe those type-level constructs could be improved with const generics in the future.