* New 'path' module under nu-cli.
Added normalize and canonicalize method.
Added some unit tests.
* Replace old usages of normalize and canonicalize.
* Fix reading symlinks and existence logic.
* Better explained
This improves incremental build time when working on what was previously
the root package. For example, previously all plugins would be rebuilt
with a change to `src/commands/classified/external.rs`, but now only
`nu-cli` will have to be rebuilt (and anything that depends on it).
