Support arrays in make_shared and allocate_shared (P0674R1)

[zoecarver]

Summary

This patch implements P0674R1. It might be a bit rough around the edges, but I wanted to get it out so it can be reviewed and I can make changes to it as I straighten out the last few parts.

The main changes are how __shared_ptr_pointer deallocates itself and (obviously) the added overloads.

• Relevant section of the standard.
• Paper.
• Based on D62259.

Thanks for the help @mclow.lists and @EricWF

[zoecarver]

include/memory

4637

Oops, I will fix this.

4740

I couldn't find a better way to do initializer_lists. Let me know if there is one.

[zoecarver]

include/memory

4686

This might create problems with scary pointers. Pointer traits should be used.

[zoecarver]

I think D62760 is a good stepping stone for this patch because of the implementation similarities. Once it has been reviewed, I can update this patch with the changes.