Reduce Position#parent use by 50%+

[Reinmar]

📝 Provide a description of the improvement

This is a performance graph for undoing removal of the "long (semantic)" content in http://localhost:8125/ckeditor5/tests/manual/performance/setdata.html.

As you can see Position#parent is the most expensive thing here. We could not find ways to make it much faster by changing its implementation.

However, then I noticed when it's used and after looking at implementation of #nodeAfter, #nodeBefore, and #textNode I realised that we're executing the #parent getter many times per each of these prop getters calls.

You can find the current implementation of these functions here: https://github.com/ckeditor/ckeditor5-engine/blob/ff045099fdea35c2af90a6da3d1a546845aeb3ea/src/model/position.js#L199-L242

Let's see if we can save 50%+ of time by rewriting them into uglier, but more optimal forms :).

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[Reinmar]

After playing with the code for 15 minutes I got down to this:

From 1165ms to 378ms :).

The entire undo action got down from 3.2s to 2.2s.

PR's coming!

[Reinmar]

Loading "long (semantic)" content into the editor:

• #parent: 2300ms -> 995ms
• entire action: 5.5s -> 4.0s