HashMap when keys are Objects that override toHash and/or opEquals

[n8sh]

Example of failure:
https://run.dlang.io/is/MQZ4Ye

/+dub.sdl:
dependency "vibe-d" version="~>0.8.5"
+/
void main()
{
    import vibe.utils.hashmap;
    import vibe.internal.allocator;

    final class Integer : Object {
        public const int value;

        this(int x) @nogc nothrow pure @safe { value = x; }

        override bool opEquals(Object rhs) const @nogc nothrow pure @safe {
            if (auto r = cast(Integer) rhs)
                return value == r.value;
            return false;
        }

        override size_t toHash() const @nogc nothrow pure @safe {
            return value;
        }
    }

    auto hashMap = HashMap!(Object, int)(vibeThreadAllocator());
    foreach (x; [2, 4, 8, 16])
        hashMap[new Integer(x)] = x;
    foreach (x; [2, 4, 8, 16])
        assert(hashMap[new Integer(x)] == x);
}

Related PR #2292.