JSON immutability

[LisiLisenok]

SDK ceylon.json module contains just mutable JSON object / array. But sometimes immutability is prefered. For example, when JSON is passed and stored with some state or options.

I would propose to incorporate immutability to the module. I've made some example / my vision how it can be done based on SDK module. See it here https://github.com/LisiLisenok/herd.json