Adds separate_* functions

[orgadish]

• Added separate_* functions to longer_wider: Request to add the separate_ functions to log_longer_wider #62
• Also refactored the tests in test_longer_wider to be easier to extend to these three new functions.

[elbersb]

nice!

• the tests are failing because of missing docs. can you run the script generate_docs.R?
• feel free to also add an item to NEWS.md

[elbersb]

looks great, thanks!