[Issue 18779] New: StatsCollector empty doesn't take into account the parent allocator

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18779

          Issue ID: 18779
           Summary: StatsCollector empty doesn't take into account the
                    parent allocator
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: trivial
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: edi33416 at gmail.com

Currently, std.experimental.allocator.building_blocks.StatsCollector does not
forward to the parent allocator, but instead, makes an assumption based on the
`bytesUsed` option.

`empty` is not the StatsCollector's decision to make, since the parent
allocator might have a different logic based on empty, than just matching
allocations with deallocations.

Please see the example below; adding the unittest in stats_collector.d will
trigger the assert

```
@system unittest
{
    import std.experimental.allocator.mallocator : Mallocator;
    import std.typecons : Ternary;

    static struct MyAlloc
    {   
        pure nothrow @safe @nogc
        Ternary empty()
        {   
            return Ternary.no;
        }   

        enum uint alignment = platformAlignment;
        void[] allocate(size_t n) { return Mallocator.instance.allocate(n); }
        bool deallocate(void[] b) { return Mallocator.instance.deallocate(b); }

        static MyAlloc instance;
    }   

    StatsCollector!(MyAlloc, Options.all) a;
    auto buf = a.allocate(42);
    a.deallocate(buf);
    assert(a.empty == Ternary.no); // fails
}
```

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