Store version information as a delta in Module::CoreList

Store version information as a delta in Module::CoreList

This reduces the size of the CoreList.pm file by storing the difference
between the versions of perl rather than a full list for each version.

In order to achieve this without changing the interface each key in the
%version hash is tied to a special tied hash that inflates the stored
difference data to the full list of modules and versions.

As part of the upgrade I ran a comparison with the
%version hash in the current Module::CoreList -- there are no
differences.