Macro derive_tests

macro_rules! derive_tests {
    ($sols:ty, $day:expr) => { ... };
}

Derive test cases for a day’s solution.

In order, the parameters are:

• Your solution type.
• The day to generate test cases for.

This expands out into a new module called tests, which can cause conflicts if you want to derive tests for multiple solutions in the same file. Consider breaking up your solutions into separate modules if you encounter this issue.