Add a Decoder

[yutotakano]

Hello Markus!

I found your library while searching for OPUS bindings for Haskell. Your encoder has been really useful.

As I see it, the code only has an Encoder, but I'd love to decode some OPUS audio as well. So here's a pull request!

Changes in this PR

• Added Codec.Audio.Opus.Decoder, which mirrors the Encoder variant for the most part, but also all specs in the OPUS API PDF
• Added Codec.Audio.Opus.Decoder.Conduit, which mirrors the Encoder variant closely.
• Added test vector tests for the new decoder, which mirrors the run_vectors.sh file in the OPUS repo

Decoder tests

The tests require opus_compare executable in the project root (which can be obtained by compiling this file), and the new opus test vectors (https://opus-codec.org/static/testvectors/opus_testvectors-rfc8251.tar.gz), but once those are in place, passes properly:

I would've loved to add similar tests for the Encoder, but the original run_vectors.sh only had decoder testing so I haven't pursued further.

I hope you can take a look/review it, and hopefully merge it in!

[yutotakano]

Whoops, noticed something, converting to a draft temporarily.

[yutotakano]

Fixed, ready for review!