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/helpers.h

@@ -93,4 +93,4 @@
 // (Having two instances of `arr` is OK because the contents of `sizeof` are not evaluated.)
 #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof *(arr))
 
-#endif /* HELPERS_H */
+#endif // HELPERS_H