exercises(space-age): overhaul to use enum

[ee7]

Closes: #405

[ynfle]

Non-blocking suggestion: Perhaps we keep string and use it to teach parseEnum and to check if it's one of them

[ee7]

Mmm. Interesting idea, but I'll keep enum here. I think that:

• the concept of "planets in the solar system" is best represented by an enum
• more type safety is better
• specifically teaching parseEnum is probably better left for a targeted concept exercise
• using enum here gives a better opportunity to practice enum-indexed arrays.