Replace custom json parser with library

[NathanJPhillips]

There is a custom json parser in util/json.h & util/json.cpp where plenty of tried-and-tested and more complete json libraries exist out there. I have personally made use of the json parser in cpprestsdk available here: https://github.com/Microsoft/cpprestsdk/blob/master/Release/include/cpprest/json.h

[trtikm]

We already made a dependency to boost., where already is a complete JSON parser.

[kroening]

CBMC is used by outsiders, and we want to enable and support this. Experience suggests to be very conservative with external dependencies, as build is a serious barrier for many. We already got problems with libzip/zlib.

[thomasspriggs]

As nice as it would be from a maintenance point of view to be using a standard library, it would add to the difficultly of building CBMC. We also don't have time allocated to do the rewriting which would be required to use a new library instead of the existing interface. I am therefore closing this out as part of a open issues clean-up effort and due to the aforementioned reasons.