[Issue 15509] IAllocator must be exposed via a reference counted struct

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=15509
Issue 15509 depends on issue 13972, which changed state.

Issue 13972 Summary: Make scoped, Unique, and RefCounted @nogc
https://issues.dlang.org/show_bug.cgi?id=13972

           What    |Removed                     |Added
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             Status|NEW                         |RESOLVED
         Resolution|---                         |MOVED

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