Tell serialize fn if it's being used on arguments

[Zenith00]

Change:
Supplies a flag to the serialize fn if it's being used on arguments.

Reasoning:
Makes it easier for users to provide different logic for serializing arguments than results. I'd like to perform some custom logic on the argument serialization that doesn't apply to result serialization.

Just adding an argument here to maximize backwards compatibility. I think this is the simplest change to allow the above, though slightly more elegant options might be to supply separate functions, etc.