[Issue 13983] RefCounted needs to be pure, @safe, nothrow

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=13983

--- Comment #6 from Walter Bright <bugzilla at digitalmars.com> ---
Partial fix (@safe not done) pulled here:

https://github.com/dlang/phobos/pull/4832

--