path2: Update based on more review feedback

Standardize the is_sep() functions to be the same in both posix and
windows, and re-export from path. Update extra::glob to use this.

Remove the usage of either, as it's going away.

Move the WindowsPath-specific methods out of WindowsPath and make them
top-level functions of path::windows instead. This way you cannot
accidentally write code that will fail to compile on non-windows
architectures without typing ::windows anywhere.

Remove GenericPath::from_c_str() and just impl BytesContainer for
CString instead.

Remove .join_path() and .push_path() and just implement BytesContainer
for Path instead.

Remove FilenameDisplay and add a boolean flag to Display instead.

Remove .each_parent(). It only had one caller, so just inline its
definition there.