Access modifiers in Scala 3

[seoethereal]

One thing missing from the Scala 3 upgrade #226 is that protected[this] and private[this] have been dropped in Scala 3 but they are somewhat prevalent across the whole repository. I think, though I am not exactly sure, that issue #231 is loosely related to variance issues caused by these access modifiers but in that case, the fix was more benign. Regardless, there must be a way to migrate from these deprecated access modifiers in the near future.