Split and rejoin tags for read/modify/write round trips

[uklotzde]

Adds a new trait SplitAndRejoinTag with methods that are needed for safely and efficiently implementing read/modify/write round trips when synchronizing metadata:

• fn split_tag(&mut self) -> Tag;
• fn rejoin_tag(&mut self);

The central function from my code base that accomplishes the task demonstrates how they are supposed to be used:

fn split_export_rejoin_track_to_tag<T>(
    tag_repr: &mut T,
    config: &ExportTrackConfig,
    track: &mut Track,
    edit_embedded_artwork_image: Option<EditEmbeddedArtworkImage>,
) where
    T: SplitAndRejoinTag,
{
    // Split the generic tag contents from the underlying representation.
    // The remainder will remain untouched while modifying the generic tag.
    let mut tag = tag_repr.split_tag();
    // Export the metadata of the given track into the generic tag, i.e.
    // add, modify, or delete items and pictures accordingly.
    export_track_to_tag(&mut tag, config, track, edit_embedded_artwork_image);
    // Rejoin the generic tag contents back into the remainder of the underlying
    // representation.
    tag_repr.rejoin_tag(tag);
    // Depending on `T` some post-processing might be required in the outer context
    // to update contents in `tag_repr` that are not (yet) supported by the generic
    // `lofty::Tag` representation.
}

Ideally the task would be implemented using a type-state-pattern that consumes its arguments to prevent mutating the remainder between split and rejoin. I am considering this alternative, stay tuned... Didn't turn out as helpful. I left a comment in the docs in case anyone has the same idea.

[Serial-ATA]

This is a good idea! Thanks for implementing it. I just have some doc changes.

[Serial-ATA]

Having thought about it, I think rejoin_tag should actually be merge_tag. Since you could really use it to add any tag into another.