sandbox r47, exposing top_cref v. emulating require

Finally, some big developments for the sandbox.  I've got it running RubyGems!

  box = Sandbox.new :init => [:all] 
  box.eval %{
    require 'rubygems'
    Gem.manage_gems
    p Gem.source_index
  }
  
This revision requires you to patch 1.8.5 to expose top_cref. (see attached)

  svn co http://code.whytheluckystiff.net/svn/sandbox/trunk sandbox

The reason for swapping top_cref is because it is placed into ruby_cref by Ruby
when any rb_load happens.  So, you end up with the right `main`.  I had thought
about emulating `require`, but it seems foolish to emulate some things and swap
others.  If I keep a list of everything that needs to be swapped, then the
procedure for native sandboxes in Ruby is known.

Anyway, emulating require isn't a good option, since the sandbox should be able
to load shared objects, if initialized to allow it.

Also, I'm using Object.module_eval(str) to do the initial eval.  Because the
only options are rb_obj_instance_eval and rb_mod_module_eval.  I would like
to be able to swap ruby_cref and then run rb_eval_string.

_why