* ast.h now includes Python-ast.h and node.h
* parsetok.h now includes node.h and grammar.h
* symtable.h now includes Python-ast.h
* Modify asdl_c.py to enhance Python-ast.h:

  * Add #ifndef/#define Py_PYTHON_AST_H to be able to include the header
    twice
  * Add "extern { ... }" for C++
  * Undefine "Yield" macro conflicting with winbase.h

* Remove "#undef Yield" from C files, it's now done in Python-ast.h
* Remove now useless includes in C files