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 issue though is that now that I've added array support, structFinals represents more than just one thing.

As another thought I had, would it be feasible to use separate pools for noscan allocations? Perhaps even separate pools for appendable, or noscan appendable allocations? Doing so would remove the need for the noscan, and appendable bit arrays to be allocated at all. It might also be reasonable to think about it for finalizable allocations as well, but as they would be much rarer in the typical use-case, it might not actually be worth it.