'Set::IntSpan' manages sets of integers. It is optimized for sets that have
long runs of consecutive integers. These arise, for example, in .newsrc
files, which maintain lists of articles:

alt.foo: 1-21,28,31
alt.bar: 1-14192,14194,14196-14221

A run of consecutive integers is sometimes called a _span_.

Sets are stored internally in a run-length coded form. This provides for
both compact storage and efficient computation. In particular, set
operations can be performed directly on the encoded representation.

'Set::IntSpan' is designed to manage finite sets. However, it can also
represent some simple infinite sets, such as { x | x>n }. This allows
operations involving complements to be carried out consistently, without
having to worry about the actual value of INT_MAX on your machine.