* ensure public API is only containing `run(Block&)`, all the rest is private API / implementation details
* adding some comments to class data members to quicker understand their meaning
* eliminate unnecessary `operator()(If&)` as it's not changing default behaviour of `ASTModifier`
* simplify readability of `visit(Expression&)`'s impl, also moving assert's into "isLatestStatementVarDeclOf", as this one is already ensuring exactly that.
* ctor impl's use of ReferenceCounter use shortened.
* renamed and improved `isLatestStatementVarDeclOf` to better match its meaning (especially since it's only used once)
