Message9175

I realized that is is inconsistent to preserve the referrer's list, but not the referents. The list would then list references that are actually invalid. So I reverted GlobalRef back to original state for now:

https://bitbucket.org/stewori/jynithon/commits/d8c610a8b92dc74cc284f5e1921f411bc42f283d

However I have plans to support preserving the referents too. At least optionally by a gc flag since it will be probably a little expensive, as gc would actively have to check for resurrection and possibly restore weakrefs. For this future work, GlobalRef will need the attribute-variant again, so I did not remove the corresponding id in JyAttribute, but marked it as a future feature.