Expression visitor should be independent of the generated language #481

Currently the ExpressionVisitor (and the snippet compiler) contains assumptions about the language of the generated code, but it should be independent of it.

Examples

• it is assumed that assignment to a variable is always in the variable = value form. Because of this assumption we have to know on the snippetcompiler level if we are assigning to a pointer or a value - which is another c++ dependency
• the c++ specific nullptr (or the c++ specific TypeConverter) is used in null expressions