Encode decode tests

* Make more structures Debug

Makes easier to println!-debug.

* Fix the cmake flags passed to build the decoding library

* Make sure to not call partially implemented ith

At least the sse4.1 variant is incomplete and triggers an assert.

* Do not use SIMD for TxType::IDTX ith

The implementation of it is incomplete.

* Provide an encode-decode test

Use `cargo test --features=decode_test -- --ignored` to run it.

* Enable decode_test in travis