module to dispose of variables

 Provides some basic helper functions for making sure variables go away when
 you want them to.
 .
 Currently provides /dispose as a default import. To avoid this:
 .
 use Variable::Disposition ();
 .
 In addition, /retain and /retain_future are available as optional imports.
 .
 use Variable::Disposition qw(dispose retain retain_future);
 .
 The :all tag can be used to import every available function: