Implement Clone for CallableValue

[amjoshuamichael]

CallableValue does not implement Clone. CallableValue is declared as Either<FunctionValue, PointerValue> – FunctionValue implements Clone, PointerValue implements Clone, therefore this generic Either should implement Clone. Adding this feature should be as simple as deriving Clone on line 77 of clonable_value.rs. Is there a reason this isn't implemented?

[TheDan64]

I don't think there's any technical reason why it hasn't been done