emoji-times: rework instructions

[ErikSchierboom]

According to the config.json, the exercise teaches:

• unicode-identifiers
• emoji-symbols

The exercise requires the student to define a Clock struct and to implement addition and subtract functions, which is almost identical to the clock exercise. As we don't allow porting practice exercises to concept exercises, we should change this. That said, I don't object to doing something with a clock-like type, but I would propose to:

1. Have the Clock type already be defined in the stub
2. Not have separate + and - functions, but just have the + function
3. Simplify by not dealing with integer overflow (that can be then practiced in the practice exercise)
4. List the emojis with their clock values in the instructions