Expose AVIndexEntry

[Queuecumber]

This PR exposes the AVIndexEntry structure and adds a pythonic container for it

The AVIndexEntry structure indexes (some or all) frames in a file, the exact behavior is demuxer dependent.

The primary use case for this is fast multi-seek loops where a video is decoded at a lower framerate than it should be displayed at.

For example if you want to decode every Nth frame, you can use search_timestamp to figure out if there is a keyframe between the frame you are currently at and the frame you want to decode and seek appropriately. If you always seek you risk seeking backwards and if you never seek you will decode a bunch of frames you didn't need.

Some relevant docs from ffmpeg:

• AVIndexEntry
• av_index_search_timestamp
• av_index_get_entry

Questions:

1. Do the unit tests look sufficient? I tried to test some "semantics" of different containers as well as the raw functionality we're adding
2. Should there be more documentation?