clippy reports unused imports when using both requires and ensures

[robjtede]

see https://gitlab.com/karroffel/contracts/-/issues/18

[robjtede]

The thing about this one is that any combination of attributes are inextricably linked because the code they generate is not strictly additive. Each attribute does a scan for the other attributes.

Making this more intitive for users would require a change of syntax that uses a single attribute. E.g.,

#[contract(
  requires( n != T::zero() || d != T::zero() )],
  ensures( ret != T::zero() && n % ret == T::zero() && d % ret == T::zero()),
)]
pub fn euclidean<T>(n: T, d: T) -> T