[Expression propagation Call] incorrect propagation of return value

[github-actions]

closes #263

[rihi]

This is a relative easy fix for #263, by making sure ExpressionPropagationFunctionCall::_is_call_value_used_exactly_once checks that a variable is not used multiple times by an instruction.

Initially, I wanted to use DataflowObject::requirements for the fix, but that method removes duplicated variables, making it not possible to check if a variable is used multiple times. Additionally, DataflowObject::requirements is defined as a @property, meaning I couldn't just simply add a default parameter like distinct=True, to control the behavior of removing duplicates.
Because that meant I had to re-implement that behavior into a new method basically doing the same thing, I decided to create a new visitor RequiredVariablesVisitor which would do that.
Because I did not want to fix every callsite of DataflowObject::requirements, so I kept that method, but changed it to delegate to the new visitor.

[rihi]

Because removing the dependency from Expression -> RequiredVariablesVisitor as required, would mean removing the DataflowObj.requirments instance method and replacing it with a 'static' method, I decided to move the visitor logic back into the DataflowObj type hierachy.
Alternatively, each call-site of DataflowObj.requirments could have been changed to use the visitor. (Perhaps via a static helper method). I figured that we don't want to do this, because it would require modifying a lot of different unrelated files, as requirements are use quite often. Additionally it would probably be weird that all kind of properties are defined, and therefore acessed, on the DataflowObj itself, while for variable requirments, one would have to use a static helper method.