String&& is just not very practical. Also return const String& when the
returned string is a member variable. The call site is free to make a copy
if he wants, but otherwise we can avoid the retain count churn.
We should work towards a pattern where we take StringView as function
arguments, and store String as member, to push the String construction
to the last possible moment.
Also run it across the whole tree to get everything using the One True Style.
We don't yet run this in an automated fashion as it's a little slow, but
there is a snippet to do so in makeall.sh.
