feat(extract): extract C/C++ preprocessor macros as Macro nodes

#define directives were not represented in the graph. Object-like macros
(preproc_def) and function-like macros (preproc_function_def) are now emitted
as Macro nodes with a DEFINES edge from the file. A function-like macro keeps
its parameter list as the signature; the macro body is not descended into.
Applies to all C-preprocessor languages (C, C++, CUDA, GLSL, Objective-C, ISPC).

Closes #375