Add `__str` cast operator

[gctucker]

In the same way that we have the __int operator to cast values passed in an API query to integers, we should have __str to cast values to strings (or not cast it to anything else). This is typically for cases where the value is only digits and would otherwise automatically be cast to an integer. It's likely to arise with the arbitrary data stored in Node.data where we can't infer the type of the fields on the API side.