Refactor mutable field in EqualsMethodUsage

[Laurens-W]

What's changed?

Refactored mutable field to be scoped better

Anyone you would like to review specifically?

@knutwannheden

Checklist

• I've added unit tests to cover both positive and negative cases
• I've read and applied the recipe conventions and best practices
• I've used the IntelliJ IDEA auto-formatter on affected files

[timtebeek]

The motivation here is that it got flagged by our best practices recipes, where we do not want mutable static fields in recipes.

Following this change it should already no longer get flagged, so then it's merely a question of whether we want to keep this field in the recipe as a static field, or move it to every visitor instance. In the latter case it might not even make much sense anymore to store the J.Binary as a field, but I don't know enough of the motivation to cache this field and any implications on performance.

[timtebeek]

Closing this PR due to the uncertainties and lack of actionable feedback.