Enable debug_print() in GUIslice core for Arduino

[ImpulseAdventure]

The debug_print() macro in GUIslice.h should be revised to support output to Serial (for Arduino targets) from errors occurring in the GUIslice core. Currently, the macro will work for the user program and GUIslice_drv_adagfx.cpp but not GUIslice.c. The primary issue is that Serial.println() will be reported as "not defined" because it is a C file and not a C++ file. Renaming GUIslice.c to GUIslice.cpp would likely work but is an undesirable workaround. Perhaps we can provide a wrapper function for Serial output that is available from the C compiled file.

[ImpulseAdventure]

Debug functionality for Arduino now implemented in release v0.8.3. Enabled with call gslc_InitDebug(). Equivalent functionality for ARM Cortex to follow. A more memory-efficient implementation is in development.