Each of these strings would previously rely on StringView's char const*
constructor overload, which would call __builtin_strlen on the string.
Since we now have operator ""sv, we can replace these with much simpler
versions. This opens the door to being able to remove
StringView(char const*).
No functional changes.
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.
