Some of the uses for the formerly supported regular expression constraints
were simply to limit values to those in a set of allowed values.
This commit adds support for that kind of simple enumeration constraint.
Regular expression constraints have caused nothing but trouble due to the
lack of a ubiquitous regular expression library. PCRE is *almost*
everywhere, but it has different versions, and different features, and
different bugs, in different places. It is more trouble than it is worth.
So this commit drops support.
