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/error.h

@@ -26,4 +26,4 @@ _Noreturn void errx(char const NONNULL(fmt), ...) format_(printf, 1, 2);
 }
 #endif
 
-#endif /* RGBDS_ERROR_H */
+#endif // RGBDS_ERROR_H