Revert rename of Div to Quot

After much discussion on IRC and #4819, we have decided to revert to the old naming of the `/` operator. This does not change its behavior. In making this change, we also have had to rename some of the methods in the `Integer` trait. Here is a list of the methods that have changed:

- `Quot::quot` -> `Div::div`
- `Rem::rem` - stays the same
- `Integer::quot_rem` -> `Integer::div_rem`
- `Integer::div` -> `Integer::div_floor`
- `Integer::modulo` -> `Integer::mod_floor`
- `Integer::div_mod` -> `Integer::div_mod_floor`