feat: extend and refactor when-then-otherwise functionality

- Wrap literals passed to `then`, `otherwise` in `alt.value`
  - opt-out as default
  - added overloads
- Wrap kwargs passed to `when` as `alt.datum.key == (value)`
- Accept multiple predicates in `when`, reducing all predicates as an `AND` reduction.
- Refactor to reduce code duplication
- Added some initial docs