ID3v2: Turn Frame into an enum

[Serial-ATA]

This is a major change to Frame that merges the header contents into the frame type itself (ex. TextInformationFrame now holds its FrameId and FrameFlags). Previously, Frame was defined as:

struct Frame { id: FrameId<'a>, value: FrameValue, flags: FrameFlags, }

Meaning that a comment would be stored as:

Frame { id: FrameId::Valid(Cow::Borrowed("COMM")), value: FrameValue::Comment(CommentFrame { .. }), flags: FrameFlags::default() }

This design required us to then check both the value and id. So, if we wanted to find all comments we would have to do:

match frame { Frame { id, value: FrameValue::Comment(..), ..} if id.as_str() == "COMM" }

With Frame now being an enum, each frame type is responsible for its ID and flags, meaning we can hide away the ID for frames that are well known:

// No longer need to specify the ID of "COMM" anywhere let frame = Frame::Comment(CommentFrame::new( TextEncoding::UTF8, *b"XXX", String::new(), String::from("Foo comment") ));

And, we can easily find all comments without having to check IDs anymore:

// We know that all `CommentFrame`s are a "COMM" frame match frame { Frame::Comment(comment_frame) => {}, }

For frames that need an ID specified, like TextInformationFrame, it is just specified in its constructor:

let frame = Frame::Text(TextInformationFrame::new( FrameId::Valid(Cow::Borrowed("TALB")), TextEncoding::UTF8, String::from("Foo album") ));