Safe wrappers around AVIOContext

[xd009642]

I'm writing some code that's making use of AVIOContext within an AVFormatContext and using a bunch of unsafe code as a result. It would be nicer ergonomically if some wrappers around this type existed in ffmpeg-next. I know this is potentially more involved as a feature but hopefully we can utilise some of the code I'm implementing now (once it's fully working).