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/pal_sorting.hpp

@@ -29,4 +29,4 @@ void rgb(std::vector<Palette> &palettes);
 
 }
 
-#endif /* RGBDS_GFX_PAL_SORTING_HPP */
+#endif // RGBDS_GFX_PAL_SORTING_HPP