System.File: change default (no 2nd arg) behavior of rmdir()
add changelog about System.File.rmdir()'s incompatible change
System.File: rename several functions

* move_file() -> move()
* move_file_exe() -> move_exe()
* move_file_pure() -> move_vim()
* copy_file() -> copy()
* copy_file_exe() -> copy_exe()
* copy_file_pure() -> copy_vim()
add changelog
Merge remote-tracking branch 'origin/rename-rmdir-2' (close #51)
Merge remote-tracking branch 'origin/system-file-rename-functions' (close #91)