Update expression-evaluation exercise: more patterns, more enums

[djmitche]

This modifies the exercise to use the standard Result type, based on feedback that students could understand it sufficiently at this point in the course.

Addresses #1565.

[marshallpierce]

It's no longer just numerical

[hurryabit]

I think using Result from std is a good idea. I know you're trying to make the expression language more complex to have more pattern matching. Without that, the solution still would have two cases of pattern matching:

1. Figure out if an expression is a binary operation or just a number literal (or atom).
2. Figure out the operation used.

IMO, that's enough. Making it more complicated risks confusing the students without gaining any more insights.

[mgeisler]

IMO, that's enough. Making it more complicated risks confusing the students without gaining any more insights.

For context, Martin is currently teaching the 4-day class here in Zurich, so this seems like valuable feedback to me.

[mgeisler]

It also uses the standard Result type, based on feedback that students could understand it sufficiently at this point in the course.

Very nice!