Write solutions after multiple exercise were completed

[LostNulide]

If multiple exercises are completed, only the currents exercise solution will be written to disk.
check_all_exercises_impl will run and detect the next exercise, but to get the solution one has to select a previous exercise and will get it afterwards.
My fix writes the solution of all exercises marked as done in check_all_exercises_impl.

Steps to reproduce:

1. Create a fresh base with rustlings init
2. Enter the directory
3. Run rustlings
4. Finish multiple exercises
5. Then go back to the terminal where rustlings is running and press n

Expected Result:

The solutions of all finished exercises are written to disk.

Actual Result:

Only the last exercises solution will be written to disk.