Tie::RefHash subclass with weakened references in the keys

 Tie::RefHash can be used to access hashes by reference. This is useful when
 you index by object, for example.
 .
 The problem with Tie::RefHash, and cross indexing, is that sometimes the
 index should not contain strong references to the objecs. The internal
 structures of Tie::RefHash contain strong references to the key, and provide
 no convenient means to make those references weak.
 .
 This subclass of Tie::RefHash has weak keys, instead of strong ones. The
 values are left unaltered, and you'll have to make sure there are no strong
 references there yourself.