[Issue 13972] Make scoped, Unique, and RefCounted @nogc

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=13972

--- Comment #7 from Nick Treleaven <nick at geany.org> ---
RefCounted and SafeRefCounted are @nogc.
Unique is @nogc (but doesn't manage heap memory anyway).
scoped is not.

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