Make comments more consistent

- Changes most `/* comments */` to `// comments`
- Changes `/**` block comments consistently to `/*`
- Adds consistent license comments to all files

Also renames `T_POP_SET` to `T_Z80_SET`

include/gfx/process.hpp

@@ -11,4 +11,4 @@
 
 void process();
 
-#endif /* RGBDS_GFX_CONVERT_HPP */
+#endif // RGBDS_GFX_CONVERT_HPP