[Api change] Merge ReferenceVariable and TemporaryVariable

Right now, we are using two Python classes to represent either a temporary variable or a reference. We did this because we assumed it would be easy to understand which type of variables we would manipulate at creation. However, over time, this has become more challenging. For example, when using `this.something.selector`, the nested member operation leads to the creation of reference variables, while what we want is a temporary variable.

I think we can evaluate merging the two classes into one, and use sub fields instead. This would allow us to easily dynamically change the type of a variable, based on the results of different analysis.