This is a prototype for adding a FINALIZE block to Perl classes to support the idea of closed classes.

This is similar to other special blocks in Perl (BEGIN, INIT, etc.) expect that it is not specific to a compiler phase or a particular compilation unit, but instead acts on the package level.

Upon importing Package::Finalize it is possible to have a number of FINALIZE blocks in your package. In the UNITCHECK phase of that particular package compilation, the stacked callbacks from FINALIZE will be called in order, after which we go about closing up the class.

A closed class is basically a class whose stash (symbol hash) has been locked, which means no entries can be added or removed from it. We make some allowances for special cases introduced by Perl's method caching mechanism, but do our best to prevent all outside manual modifications.

Dunno yet, but this is something that might be needed by the p5-mop work I am doing.