v3.0.0 - suggestion - Add context argument to callbacks

[zfields]

By adding an additional argument, void * context, to the callbacks, the callback provider can associate the callback to different endpoints. This would enable multiple streams to be managed simultaneously by the callback provider. If the user does not need or desire the context, then they will be able to provide a NULL to the context argument.

Current Implementation:

extern "C" {
  // callback function types
  typedef void (*callbackFunction)(uint8_t, int);
  typedef void (*systemCallbackFunction)(void);
  typedef void (*stringCallbackFunction)(char *);
  typedef void (*sysexCallbackFunction)(uint8_t command, uint8_t argc, uint8_t *argv);
}

Proposed Implementation:

extern "C" {
  // callback function types
  typedef void (*callbackFunction)(uint8_t, int, void *);
  typedef void (*systemCallbackFunction)(void *);
  typedef void (*stringCallbackFunction)(char *, void *);
  typedef void (*sysexCallbackFunction)(uint8_t command, uint8_t argc, uint8_t *argv, void *context);
}

[zfields]

This may be closed, unless you want to provide context at the FirmataClass level.