The Circuit trait is extended with the following:
```
pub trait Circuit<F: Field> {
    /// [...]
    type Params: Default;

    fn params(&self) -> Self::Params {
        Self::Params::default()
    }

    fn configure_with_params(meta: &mut ConstraintSystem<F>, params: &Self::Params) -> Self::Config {
        Self::configure(meta)
    }

    fn configure(meta: &mut ConstraintSystem<F>) -> Self::Config;
}
```

This allows runtime parametrization of the circuit configuration.  The extension to the Circuit trait has been designed to minimize the breaking change: existing circuits only need to define the associated `type Params`.

Unfortunately "Associated type defaults" are unstable in Rust, otherwise this would be a non-breaking change.  See rust-lang/rust#29661