We have started to use Gadget fields in a project and for most of them, we have to exchange more than 1 text value with the client part. Currently, our TALES expressions are polluted with modules["json"].dumps(...), plus JSON.parse calls on client side.

But the most important change in this commit is that it encourages to use proper serialization when needed (it's a common trap to do quick&dirty serialization, for example by joining/splitting with a particular char).

It's a small change and I doubt there would be a choice than JSON here.

/cc @romain