Do ext checks after translation.

[sanket1729]

Fixes #238

Also adds some basic tests

[apoelstra]

I think the CI failure is something real, though maybe it means that we need to update the test.

[tcharding]

The bug is in the test I believe, just change the match statement in ms_attributes_test to be:

        match (ms, valid) {
            (Ok(ms), true) => {
                assert_eq!(format!("{:x}", ms.encode()), expected_hex);
                assert_eq!(ms.ty.mall.non_malleable, non_mal);
                assert_eq!(ms.ty.mall.safe, need_sig);
                assert_eq!(ms.ext.ops.op_count().unwrap(), ops);
            }
            (Err(_), false) => {}
            (Ok(_), false) => panic!("false positive"),
            (Err(_), true) => panic!("false negative"),
        }

[tcharding]

Here is a refactor of the new unit test I did while debugging if you want it:

    // Sanity test for `translate_fails_for_duplicate_key`.
    #[test]
    fn can_translate_different_keys() {
        let ms = Miniscript::<String, Segwitv0>::from_str("and_b(pk(A),a:pk(B))").unwrap();
        let mut t = StrKeyTranslator::new();

        let key0 = bitcoin::PublicKey::from_str(
            "0238e6a8035e67d46f2a350f748a9c2dd45ba467f12432e5f9371ca91fb9831086",
        ).unwrap();
        // insecure complement of key0
        let key1 = bitcoin::PublicKey::from_str(
            "03a8e6a8035e67d46f2a350f748a9c2dd45ba467f12432e5f9371ca91fb9831086",
        ).unwrap();

        t.pk_map.insert(String::from("A"), key0);
        t.pk_map.insert(String::from("B"), key1);
        ms.translate_pk(&mut t).expect("translation failed");
    }

    #[test]
    fn translate_fails_for_duplicate_key() {
        let ms = Miniscript::<String, Segwitv0>::from_str("and_b(pk(A),a:pk(B))").unwrap();
        let mut t = StrKeyTranslator::new();

        let key = bitcoin::PublicKey::from_str(
            "0238e6a8035e67d46f2a350f748a9c2dd45ba467f12432e5f9371ca91fb9831086",
        ).unwrap();

        t.pk_map.insert(String::from("A"), key);
        t.pk_map.insert(String::from("B"), key);
        match ms.translate_pk(&mut t) {
            Err(TranslateErr::OuterError(Error::AnalysisError(AnalysisError::RepeatedPubkeys))) => {},
            Err(_) => panic!("Unexpected error"),
            Ok(_) => panic!("False positive"),
        }
    }