text: Title: Add `drop_tables` function
  Body: We recently added a `create_tables` function:
  
  https://github.com/piccolo-orm/piccolo/blob/bded0f62c6aa14ed4ac294cd560f56c6dd160f2e/piccolo/table.py#L978
  
  It is a great convenience when writing unit tests, or writing a simple data science script using Piccolo.
  
  ```python
  create_tables(Band, Manager)
  ```
  
  It would be nice to have a companion function called `drop_tables`. It will sort the tables based on `ForeignKey` columns, and drop them in the correct order (or else just call `drop_table(cascade=True)`. The implementation will be very similar to `create_tables`.
  
  ```python
  drop_tables(Band, Manager)
  ```