path2: Replace the path module outright

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.
path2: Update asserts for new format!() style
path2: Update for loop -> continue
path2: Write a few paragraphs of module documentation
path2: Remove Path::normalize()

There are no clients of this API, so just remove it.

Update the module docstring to mention normalization.