LoadFunction::signature's documentation does not match it's uses

[doug-q]

documentation: "The instantiation of func_sig".

Here, instantiation means the Signature of the target polymorphic function after the type args of the LoadFunction op are substituted.

it is used as the the signature of the op.

This new function on impl LoadFunction would give the instantiation:

    pub fn instantiation(&self) -> Signature {
        let TypeEnum::Function (ty) = self.signature.output[0].as_type_enum() else {
            panic!("")
        };
        ty.as_ref().clone().try_into().unwrap()
    }

I suggest that rather than implement the above function, we rename signature -> instantiation, and that all uses are changed to call DataflowOpTrait::signature() which would be redefined to compute the signature from the instantiation.