Unified path expansion under new module and better canonicalize (#1571)

* 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

crates/nu-cli/src/lib.rs

@@ -17,6 +17,7 @@ mod evaluate;
 mod format;
 mod futures;
 mod git;
+mod path;
 mod shell;
 mod stream;
 mod utils;