cpp/experimental/fs/path/append

@1@ First, appends the preferred directory separator to, except if any of the following conditions is true: @@* the separator would be redundant ( already ends with a separator) @@* is empty, or adding it would turn a relative path to an absolute path in some other way @@* is an empty path. @@* begins with a directory separator. @@Then, appends to the pathname maintained by @2,3@ Same as, but accepts any , null-terminated multicharacter string, or an input iterator pointing to a null-terminated multicharacter sequence. @4@ Same as, but accepts any iterator pair that designates a multicharacter string.