This allows deriving Debug on user-defined structs that contain
instances of EcGroup.

For standard groups, the debug output will look something like this:

    EcGroup { curve_name: "secp521r1" }

For groups not based on a standard curve, the Debug impl will try to
extract and print the curve's components, such that:

    let mut p = BigNum::new().unwrap();
    let mut a = BigNum::new().unwrap();
    let mut b = BigNum::new().unwrap();
    let mut ctx = BigNumContext::new().unwrap();

    EcGroup::from_curve_name(Nid::SECP224R1)
        .unwrap()
        .components_gfp(&mut p, &mut a, &mut b, &mut ctx)
        .unwrap();

    // reconstruct the group from its components
    let group = EcGroup::from_components(p, a, b, &mut ctx).unwrap();

    println!("{:#?}", group);

will print:

    EcGroup {
        p: "FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF000000000000000000000001",
        a: "FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFFFFFFFFFFFFFFFFFFFE",
        b: "B4050A850C04B3ABF54132565044B0B7D7BFD8BA270B39432355FFB4",
    }

If EcGroup::components_gfp() fails, or if any other error occurs, we'll
instead print only:

    EcGroup

Calling EcGroup::components_gfp() on a curve over GF(2^m) unexpectedly
works, and there does not seem to be an explicit way to distinguish
custom curves over GF(2^m) from custom curves over GF(p), and so, custom
curves over GF(2^m) will be Debug'd by the same logic.  (rust-openssl
does not allow constructing custom curves over GF(2^m), so we can't add
test cases for this.)