change shuffle to include enum instead of integer encoding

[bjornregnell]

Many had problems with enums at the diagnostic test. One way to improve knowledge about enums is to make shuffle use them from start instead of starting off with integer encoding and then have an optional task on enums.

[jonmellerby]

This seems like a good idea to me. It's probably more educational to write good code and then reflect on why it's good, than to purposely write suboptimal code and reflect on why it's bad.