[PATCH] Add support for "fancy" pointers to promise and packaged_task.

[Marshall Clow]

LGTM

================
Comment at: include/future:788
@@ -785,2 +787,3 @@
+    __a.deallocate(_PTraits::pointer_to(*this), 1);
 }
 
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This repetitive code bothers me; both here and in D4859.
I don't see an obvious way to package it up, though.

(Yes, I know it's just types, not actual code, but if it's in multiple places in the source, there's a chance that it's wrong in one or more of them)

http://reviews.llvm.org/D4862