Remove the old path.
Rename path2 to path.
Update all clients for the new path.
Also make some miscellaneous changes to the Path APIs to help the
adoption process.
While usage of change_dir_locked is synchronized against itself, it's not
synchronized against other relative path usage, so I'm of the opinion that it
just really doesn't help in running tests. In order to prevent the problems that
have been cropping up, this completely removes the function.
All existing tests (except one) using it have been moved to run-pass tests where
they get their own process and don't need to be synchronized with anyone else.
There is one now-ignored rustpkg test because when I moved it to a run-pass test
apparently run-pass isn't set up to have 'extern mod rustc' (it ends up having
linkage failures).
The normal unit tests cannot change the current working directory because it
messes with the other tests which depend on a particular working directory.
