Add support for AMR-WB codec.

[dennwc]

Use opencore-amr for decoding and vo-amrwbenc for encoding, instead of code from 3GPP spec which has unclear licensing.

Based on #57.

[biglittlebigben]

Would it to separate the actual C code from the CGO implementation in each directory? This would make more clear what code we copied vs wrote.

It's also not realistic to review all this C code. Not sure what this means in practice if we commit it in our own repo... Maybe we should ask for guidance about best practices for such cases? Would a git submodule make any sense? Or am I overthinking this...

[dennwc]

@biglittlebigben splitting C and Go code into directories will break cgo compilation/caching. Unfortunately cgo is not flexible enough in this regard. All C and associated Go files must be co-located in the same directory.

This is why I intentionally flattened all required files from these libraries, so that cgo can build them as a single package, without any external dependencies. Otherwise it will require extra headers/libraries on the host, which we'd like to avoid.

I could split this code into a separate repo, if needed. That would work.

Long-term the plan is to replace it with a pure Go implementation, similar to how G722 was translated.

[biglittlebigben]

I can see how importing a bunch of outside code in a repo we otherwise own may be flagged in an audit, but absent a compliance person to consult with, I do not have better option to confidently suggest...

[dennwc]

I moved the C source into a separate project. This way we could quickly swap it for something else.