Support extracting macro numeric and string literal constants

[keynmol]

Currently, code like #define CONST 0x25 does not register with sn-bindgen at all.

Some libraries have a lot of constants, requiring the users of bindings to manually add the constants, e.g. https://github.com/libsdl-org/SDL/blob/SDL2/include/SDL_audio.h#L96 and https://github.com/JD557/minart/blob/master/backend/native/src/main/scala/sdl2/SDL.scala#L99-L133

It seems that if we set a particular flag during parsing, we can get access to macro definitions – then we manually reparse a list of tokens to one of the known patterns used by C libraries, and define those macro constants inline.

1. libclang flag: CXTranslationUnit_DetailedPreprocessingRecord
2. tokenisation: https://clang.llvm.org/doxygen/group__CINDEX__LEX.html#ga6b315a71102d4f6c95eb68894a3bda8a
3. The cursor type is https://clang.llvm.org/doxygen/group__CINDEX.html#ggaaccc432245b4cd9f2d470913f9ef0013aaf2a8a83171b883cabe83ef3d729e138 (CXCursor_MacroDefinition)

This feature should be delivered in steps:

1. Extract numeric constants (integral and floating point) e.g. #define AUDIO_S16LSB 0x8010
2. Extract string literals #define BLA "string"
3. Handle single back references e.g.

#define AUDIO_U16MSB    0x1010  /**< As above, but big-endian byte order */
#define AUDIO_S16MSB    0x9010  /**< As above, but big-endian byte order */
#define AUDIO_U16       AUDIO_U16LSB
#define AUDIO_S16       AUDIO_S16LSB

4. Handle arithmetic/bit manipulation logic (only allow referencing previously registered constants)

#define SDL_AUDIO_ALLOW_FREQUENCY_CHANGE    0x00000001
#define SDL_AUDIO_ALLOW_FORMAT_CHANGE       0x00000002
#define SDL_AUDIO_ALLOW_CHANNELS_CHANGE     0x00000004
#define SDL_AUDIO_ALLOW_SAMPLES_CHANGE      0x00000008
#define SDL_AUDIO_ALLOW_ANY_CHANGE          (SDL_AUDIO_ALLOW_FREQUENCY_CHANGE|SDL_AUDIO_ALLOW_FORMAT_CHANGE|SDL_AUDIO_ALLOW_CHANNELS_CHANGE|SDL_AUDIO_ALLOW_SAMPLES_CHANGE)