Readd `;;; Code:` comment to package start of solution files

[fapdash]

As mentioned in #18, Emacs is very particular about the expected structure of packages.
#210 added a regression, if we can call it that in this context.
All the tests now have function stubs, which is great, but the packages now don't follow the documentation guidelines (;;; Code: as part of the package start is missing).
My suggestion would be to

1. add the ;;; Code: comment to the start of the package again
2. follow the pattern used in Add list ops exercise #225 and to put errors in the function stub body

To make sure that the guidelines are always followed we could add checkdoc to the CI test run (#233)

edit: I realize that this might seem a bit pedantic but I think it's important to follow the conventions, so we lead people, who are learning the language, by example.