[FIXUP LATER] implemented os_file_get_size()
[FIXUP LATER] implemented FileInfo struct
[FIXUP LATER] replaced some mch_stat() with os_file_exists()
[FIXUP LATER] replaced mch_stat() in buffer.c
[FIXUP LATER] replaced last mch_stat() and mch_lstat()
[FIXUP LATER] replaced some mch_stat() with os_get_file_info()
[FIXUP LATER] replaced some mch_lstat()