Remove the majority of the XML processing logic.

This keeps the top-level compounddef processing, but removes the
rest. The intent is to rework this logic to provide a better data
model.
Remove the C++ parser logic.

The problem with the parser logic is that:
  (a) it is incomplete,
  (b) it does not follow a sensible AST model,
  (c) only the token stream is used by the documentation code.
cpp: split out the tests
cpp: fix python 3 compatibility
Rename cpp.py to cpplex.py

The cpp.py file is really a C++ tokenizer/lexer. Therefore, rename
it to match that function.