Add C++ code generation utilities

The code in cxx_gen.py is mostly from conveyor/site_scons/cxx.py with
some additional tools added to support conveyor-ui (originally
provided by conveyor-ui/site_scons/cxx_gen.py).

This will be used to replace the old cxx.py and cxx_gen.py code.

Also added an empty __init__.py so that modules from this directory
can be imported.