- borrowBook(User, Book) is added as a unified function that will
update and change all required data when a user attempts to borrow a
book

- It now returns the array of the actual numbers instead of the default
formatting

- Fixed missing semicolon on import statement
- Fixed formatting error when declaring ‘int[] currentDay’

- Added returnBook(User, Book) which will update both the users and the
books upon success on returning the book
- Added currentDay() that would return the current date in int array
- Added calDueDate(int) that takes the current date from currentDay()
and calculate the due date from there