Make Settings Send + Sync

[jyn514]

This works by requiring ElementContentHandlers to be Send + Sync. Making Settings Send + Sync can be useful if you want to reuse the same settings across multiple rewriters in different threads.

This is a breaking change.

Helps with #82, but I'd prefer to avoid merging it until I hear back from the author about why they need this.

[kornelski]

Do callbacks really need to be Send + Sync? This may be an annoying limitation, especially Sync.

[jongiddy]

This causes problems with existing code if Rc<RefCell<_>> is used to share data between multiple closures. Such structures are required for keeping state over multiple closures, especially when using on_end_tag with its closure within a closure (e.g. https://github.com/jongiddy/strictly-data/blob/b5ff138149cfb0fd763fc1c44639c81388f5ebd2/src/extract.rs#L493-L499).

If this change is merged, would such structures need to be replaced with Arc<Mutex<_>> even if threading is not used?

[jyn514]

If this change is merged, would such structures need to be replaced with Arc<Mutex<_>> even if threading is not used?

It would, yes.

Ok, I think I'm convinced this isn't the right way to fix this.