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.
path2: Remove some API functions

Delete the following API functions:
- set_dirname()
- with_dirname()
- set_filestem()
- with_filestem()
- add_extension()
- file_path()

Also change pop() to return a boolean instead of an owned copy of the
old filename.
path2: Remove Path.into_str()
path2: Update for latest master

Also fix some issues that crept into earlier commits during the conflict
resoution for the rebase.
path2: Update for privacy changes

Remove redundant `contains_nul` definition.

Make `parse_prefix` private.