Feature parity between Variable and VariableInstancers, maybe even VariableReferences

[burakomer]

Is your feature request related to a problem? Please describe.
I use Instancers a lot in my projects, for example IntVariableInstancer. I'd like to use Add, Subtract etc. methods from it as well, but they're only defined in IntVariable thus you can't call them from UnityEvents with the instancer.

Describe the solution you'd like
Simple delegation of implementation-specific methods of Variable classes in corresponding VariableInstancers.

Describe alternatives you've considered
I can't think of any. Maybe it can be implemented with interfaces for consistency-sake. Something like INumberOperations that has the methods like Add, Subtract, Divide etc, then IntVariable and IntVariableInstancer, maybe even IntReference could implement it, directly modify the value or delegate when can.