Value Iteration

I would like to start a discussion on having a class of `value_iterator`, which will be used to iterate over a `value` instance like visiting a tree in a depth-first manner.

Currently, we can iterate over `array` and `object` but I think a more general iterate functionality will be helpful for many applications. Also it would be nice that `value` supports generic iteration like other container classes.

I prefer each iteration to return a key-value pair of a JSON Pointer and value reference but I would like to ask your opinion regarding it and whether we can utilize the already implemented mechanisms/structures in the library.