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 problem with the code you have there is that it's perfectly possible for an array to be finalized from 2 threads at once, your code makes no thought of this. If however, we move the clearing of the finalize attribute above the actual finalization, we can help to avoid this issue.....~~

Then again, it really makes no difference, as we aren't actually checking to see if the struct has been finalized yet... Ah well, it would just end up as a preference then.