Small improvements to codes, directories.

* [ATOM_EXT,CODES_EXT,SC] Relocated codes_replace/4 from ATOM_EXT to CODES_EXT.
* [CODES_EXT,SC] Rewrote codes_replace/4 using DCG_GENERIC (shorter code, no append/3).
* [DIR_EXT,NEW] Added wrapper create_nested_directoy/1.
* [FILE_EXT,FIX] Syntax error.
Various extensions and fixes for handling directories and files.

* [DIR_EXT,EXT] create_personal_subdirectory/2 now returns the absolute file name of the created directory.
* [DIR_EXT,FIX] is_absolute_file_name/1 in create_nested_directory/2 would throw an exception on non-atomic input.
* [DIR_EXT,EXT] Trashcan initialization now adds `trash` to the file search path.
* [FILE_EXT,NEW] Added merge_into_one_file/2 and split_into_smaller_files/3.