I spent some time rewriting the comments and a couple other cosmetic
changes, and squashed into two patches: the second one has the
optimized string hashing. They each have still just one demo use case.
It looks pretty close to commitable, but I'll leave this up for a few
days in case anyone wants to have another look.
After this first step is out of the way, we can look into using this
more widely, including dynahash and the GUC hash.
