Proto: H-O functions' args given in consistent ordering in skip_list_repr

Context

This MR simply changes the interface of some higher-order functions in skip_list_repr module to provide them in a consistent way: the function for keys first, then the function for values/content.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR