cpp_transpiler is quite difficult to extend meaningfully

The xls/dslx/cpp_transpiler can be used to create C++ mirrors of DSLX structures and types and functions that convert between them. While it works reasonably well, it is extremely difficult to make even modest changes and improvements to the output, mostly due to it avoiding any templating language and instead building the source and headers it creates out of raw strings (in C++).

It may be worthwhile to rewrite this using the jinja templating language used for the jit wrappers and other tools.

Related: https://github.com/google/xls/issues/1135