Add annotations for the main function(s) of every exercises

[ageron]

This PR adds annotations to the main function(s) of every exercise.

This includes all functions in the main file (e.g., exercises/practice/leap/Leap.roc) and the same functions in the example code (e.g., exercises/practice/leap/.meta/Example.roc) but I did not annotate other functions (yet?).

Also note that in some cases the annotations removed the need for some error test cases: I removed the test cases for negative inputs if the input is annotated as a positive integer like U8 or U64.

Side note: I removed some commented-out code that was meant to show alternative ways of implementing things, because such code often ends up stale, plus the Example.roc file doesn't seem to appear anywhere on the website, it just looks like it's meant to ensure that the test cases work well.

[ageron]

There's a debate on Roc's Zulip Chat to decide whether the test cases for expected errors should be minimal (e.g., result |> Result.isErr or whether it should be detailed (e.g., result == Err (QuestionArgHadAnUnknownOperation "What is 52 cubed?")).

Depending on the outcome of this discussion, I might update this PR to make the annotations more or less detailed for errors:

• Now: answer : Str -> Result I64 [UnknownOperation, SyntaxError]
• Minimal: answer : Str -> Result I64 _
• Detailed: answer : Str -> Result I64 [QuestionArgHadAnUnknownOperation Str, QuestionArgHadASyntaxError Str]

I'll also update PR #64 accordingly.

[isaacvando]

Looks good!