Make symphonia_bundle_mp3::header public

[helgoboss]

I was wondering if you could make the symphonia_bundle_mp3::header module public?

Background: I need to check whether an MP3 is VBR or CBR, by looking at all frame headers via header::read_frame_header(&mut reader) (without actually decoding the frames). This works great and is fast. But I currently need to use the following simple patch:

helgoboss@12e3eec

Thanks a lot for this crate!