Duplications between llvm-backend and blockchain-k-plugin

runtime/json/json.cpp implements KoreHandler, KoreWriter, and write_json. In blockchain-k-plugin in plugin-c/json.cpp those are also being implemented, and slightly differently (I guess they're older versions?). Making a header file with declarations for these would allow the blockchain plugin to use them.

Inversely, runtime/json/json.cpp has a few structs declared which are provided by a header file in blockchain-k-plugin in plugin-c/k.h. These are zinj, stringinj, boolinj, jsonlist, json, and jsonmember, along with some extern "C" stuff. There are also declarations of global block * and blockheader constructs scattered around, even in firefly and iele-semantics. These are just typedefs and simple constructs but maybe it would be good to get them centralized somewhere with some basic utilities as a rudimentary "K API".

Start by comparing runtime/json/json.cpp with blockchain-k-plugin/plugin-c/json.cpp and plugin-c/k.h to inventory duplicated implementations and declarations. Then inspect the scattered block and blockheader declarations in firefly and iele-semantics. Done should be defined as an agreed shared set of declarations and utilities that removes the identified duplication across these components.