show typemax(VersionNumber) as v"∞" (not valid input syntax).
pkg2 wip: much simpler intersection of version sets.

This does away with the VersionSet type in favor of VersionInterval
and uses vectors of VersionIntervals to represent sets of versions,
i.e. a union of an ordered series of disjoint version intervals. The
typemin and typemax methods for VersionNumbers greatly simplify this.