Create C API

[ChunMinChang]

We should create C APIs for this crate so it can be reused in the cubeb C backends as well.

[ChunMinChang]

We should probably also compare the performance of this crate with the cubeb_mixer to see what can we improve here.

[padenot]

I wouldn't expect this mixer to show up in profiles, so it's probably not necessary to optimize it right now, but it would be good to check this assumption.

[ChunMinChang]

Now I've tried two approaches:

1. Exposing C API behind a feature flag: draft
2. Create a separated crate for C API (like what mp4parse does): draft

If the C APIs are exposed behind a feature, the pro is the C APIs are always updated to the latest version. The cons is there will be duplicated #[cfg(feature = "capi")] in the code. I don't expect the implementation of this crate getting complicated, so maybe the duplicate #[cfg(feature = "capi")] is manageable.

On the other hand, if C APIs are exposed by a separated crate, the pro is no special settings in the main mixer crate (no flags are used). The con is it needs to maintain and update/publish two crates.