Write a program to manage a library system that tracks which books are checked out and who has borrowed them. The program should allow assigning books to borrowers, returning books, and listing all currently borrowed books.

• When a book is checked out, add it to the collection with the borrower's name.
• When a book is returned, remove it from the collection.
• When listing borrowed books, return a list of strings, each showing the book title and the borrower's name.