Stream picture tags

Summary

The ability to read the picture tags as a stream for improve performance

API design

let probe = lofty::Probe::open(path);

let tags_reader = probe.into_reader();

// Consumes tags_reader and returns an iterator that skips non-picture tags
let pictures_iterator = tags_reader.pictures();

let first_picture_reader = pictures_iterator.next()?;

let bytes = picture_reader.data; // std::io::Bytes<> iterator
let mime_type = picture_reader.mime_type; // Access other info about the picture

Start by locating Probe::open and the proposed into_reader API, then compare the existing picture-tag parsing path with the requested pictures() iterator. Done means callers can iterate picture tags as readers and access each picture's byte stream and MIME type without loading all picture data at once.