This patch improves the display of functions generated by
"functions()". If the "returns=" strategy can only produce a single
value, the function is now shown as a constant lambda such as
"f=lambda x: 3", instead of noting each call to it separately.
We also respect the "repr_pretty()" method of objects which define
one in a few more places, such as the arguments of a failing example.
The canonical version of these notes (with links) is on readthedocs.