Modularize JIT

Nobody has claimed this yet.

Currently, `src/utils/generate_cxx_code.cpp` has a lot going on. We need to break it down and modularize it.
First, create a JIT structure that encapsulate all the maps, sets, vectors, and list that are passed to static functions.
Second, make the functions that populates those data structure as the member methods of the JIT structure.
Thirst, separate the routines that populates data structure from IO routines. i.e., wrap them into separate functions.
Fourth, revisit the variable name choices.
This will make the code more readable and versatile.

Start by reading src/utils/generate_cxx_code.cpp and map the static functions, shared maps, sets, vectors, and list described in the issue. Define the JIT structure around that shared state, then separate data-population routines from I/O routines and review variable names. Done means the generator is modularized with those responsibilities separated.

Written by the indexing model from the issue text.