Convert more tests to Proptest

[daxpedda]

To make the code cleaner I implemented Arbitrary for various types where appropriate. The implementation is a bit naive because it simply maps by reducing instead of properly having a min/max implementation via a custom ValueTree.

I also switched to the newer #[property_test] instead of proptest!.

Follow-up from #806.

[daxpedda]

I think this would be better split up into multiple tests, but I didn't know exactly how.

So here is a suggestion, let me know what you think:

    /// Check that mul_base_clamped and mul_clamped agree

    #[test]
    fn mul_base_clamped_max() {
        // Test that mul_base_clamped and mul_clamped agree on a large integer. Even after
        // clamping, this integer is not reduced mod l.
        let a_bytes = [0xff; 32];
        assert_eq!(
            EdwardsPoint::mul_base_clamped(a_bytes),
            constants::ED25519_BASEPOINT_POINT.mul_clamped(a_bytes)
        );
    }

    #[property_test]
    fn mul_base_clamped(a_bytes: [u8; 32]) {
        prop_assert_eq!(
            EdwardsPoint::mul_base_clamped(a_bytes),
            constants::ED25519_BASEPOINT_POINT.mul_clamped(a_bytes)
        );
    }

    #[property_test(config = ProptestConfig { cases: 50, .. ProptestConfig::default() })]
    #[cfg(feature = "precomputed-tables")]
    fn mul_base_clamped_precomputed(b: EdwardsPoint, a_bytes: [u8; 32]) {
        // Make a random curve point in the curve. Give it torsion to make things interesting.
        let random_point = b + constants::EIGHT_TORSION[1];
        // Make a basepoint table from the random point. We'll use this with mul_base_clamped
        let random_table = EdwardsBasepointTableRadix256::create(&random_point);

        prop_assert_eq!(
            random_table.mul_base_clamped(a_bytes),
            random_point.mul_clamped(a_bytes)
        );
    }