The GC will now call destructors on heap allocated structs

~~This PR is not quite ready to merge, as I need to add a set  of tests for it, and also need to get it to work on arrays of structures as well. I am creating it now to make sure that I've not gone and broken druntime terribly with my approach to fixing the issue, as well as to take feedback on my method of solving the issue.~~ And the link to the issue this solves is [here](https://issues.dlang.org/show_bug.cgi?id=2834).

In the case where a struct is a member of a class, the struct's destructor will be called after the body of the class's destructor.

This PR is now ready to be reviewed and hopefully merged. It is not feasible to implement calling destructors of structs in heap allocated arrays for various reasons, including the inability to know which elements of the array are actually initialized, as well as which elements are valid.

This requires D-Programming-Language/dmd#3727
The GC already has the GC.query information when calling these functions, and knows that attributes have to be cleared. I suggest passing the information in the callback, and wrapping it in case of explicitely calling it:

```
extern (C) void rt_finalize_array(void* p, bool resetMemory = true) nothrow
{
    if(!p)
        return;
    BlkInfo inf = GC.query(p);
    rt_finalize_array2(p, inf.size, ressetMemory);
    GC.clrAttr(p, BlkAttr.FINALIZE);
}
```

And then call rt_finalize_array2 from within the GC.