reimplement Dsa::generate in terms of generate_params/generate_key

impl Dsa<Private> {

    /// Generate a DSA key pair.

    ///

    /// Calls [`DSA_generate_parameters_ex`] to populate the `p`, `g`, and `q` values.

    /// These values are used to generate the key pair with [`DSA_generate_key`].

    ///

    /// The `bits` parameter corresponds to the length of the prime `p`.

    ///

    /// [`DSA_generate_parameters_ex`]: https://www.openssl.org/docs/manmaster/crypto/DSA_generate_parameters_ex.html

    /// [`DSA_generate_key`]: https://www.openssl.org/docs/manmaster/crypto/DSA_generate_key.html

    pub fn generate(bits: u32) -> Result<Dsa<Private>, ErrorStack> {

        ffi::init();

        unsafe {

            let dsa = Dsa::from_ptr(cvt_p(ffi::DSA_new())?);

            cvt(ffi::DSA_generate_parameters_ex(

                dsa.0,

                bits as BitType,

                ptr::null(),

                0,

                ptr::null_mut(),

                ptr::null_mut(),

                ptr::null_mut(),

            ))?;

            cvt(ffi::DSA_generate_key(dsa.0))?;

            Ok(dsa)

        }

        let params = Dsa::generate_params(bits)?;

        params.generate_key()

    }

    /// Create a DSA key pair with the given parameters