This replaces the current LexicalPath::append() API with a new method
that returns a new LexicalPath object and doesn't touch the this-object.
With this, LexicalPath is now immutable. It also adds a
LexicalPath::parent() method and the relevant test cases.
Since this is always set to true on the non-default constructor and
subsequently never modified, it is somewhat pointless. Furthermore,
there are arguably no invalid relative paths.