reference counting memory management

[nbittich]

• Introduce a new Primitive :
  Primitive::Ref(MutPrimitive)
• instead or cloning the primitive, use the ref
• for function we still clone variables passed as parameters, but we could introduce &v to allow passing reference as parameters
• Might be required: add TreeNodeValue::Variable / VariableAssign { name_assign: String, name_from_ctx: String, negate: bool } ,and refactor variable_from_ctx to evaluate the variable in compute