• Add : New script : samples/vm_vs_tie.pl, that benchmarks our uvar
  magic versus tied hashes.
  
• Add : The VMGCOMPAT* constants can be used from userspace to check
  perl magic abilities.
  
• Fix : Callbacks that returned undef made us croak, breaking the
  variable behaviour (regression test in 14-callbacks.t).
  
• Fix : uvar callbacks weren't tested for non-NULL-ity before being
  called (regression test in 28-uvar.t).
  
• Tst : Fix typo in 25-copy.t that prevented Tie::Hash tests to be ran.