SI-6054 Modern eta-exp examples that almost run

This wasn't so easy after all. The clever one is to
show that Option.fold(=>Z) preserves the by-nameness
of the zero value, which may or may not be in the
spec yet.  The addition to 6.26.5 was (I think)
rejected as implementation-specific; it should
use the wording from 6.6 about "preserving by-
nameness" or "by-namity" (the latter has the
advantage of sounding both more technical and
somewhat inane).

This supplement is no reflection on the quality of the previous edit.  But it has the virtue of using actual ordinary library functions found around the home.


Review by @adriaanm