Enhance performance of visiting parsing model

[pekkaklarck]

When Robot parses data files, it produces a model that both Robot itself and external tools use. Going through the model is easiest by using visitors. That is typically pretty fast, but it is possible to enhance the performance, for example, by caching visitor methods. These enhancements are especially important for external tools such as editor plugins that need to process lot of data.

We already have PR #4911 by @d-biehl that adds visitor method caching. This issue exists mainly for tracking purposes, but it also covers mentioning caching in visitor documentation. Possible additional performance enhancements can be linked to this same issue as well.

[pekkaklarck]

I just noticed that we actually use ast.NodeVisitor directly, not our own ´ModelVisitor, when transforming parsing models to running models. That obviously means that performance enhancements to ModelVisitor` cannot have any effect to parsing. I'll fix that.