[PMC] Converted Pair PMC to use PDD 17 attribute storage.  As part of this, it now only holds PMC keys.  If we need STRING keys, we need a new implementation. (Replace conditional with polymorphism.)
[PMC] Converted Pair PMC to use PDD 17 attribute storage.  As part of this, it
now only holds PMC keys.  If we need STRING keys, we need a new implementation.
(Replace conditional with polymorphism.)

git-svn-id: 6e74a02f85675cec270f5d931b0f6998666294a3@29779 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

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