Add #[no_drop_flag] attribute

[Aatch]

This adds a #[no_drop_flag] attribute. This attribute tells the compiler to omit the drop flag from the struct, if it has a destructor. When the destructor is run, instead of setting the drop flag, it instead zeroes-out the struct. This means the destructor can run multiple times and therefore it is up to the developer to use it safely.

The primary usage case for this is smart-pointer types like Rc<T> as the extra flag caused the struct to be 1 word larger because of alignment.

This closes #7271 and #7138

[Aatch]

cc @thestinger

[brson]

I guess this is fine but is there any plan to address this in a cleaner way? Under what situations will the dtor be called more than once?

Running dtors multiple times is pretty scary. I'm worried that aspect of this feature will be underappreciated since it isn't obvious or documented.

[thestinger]

@brson: this is a stopgap until @pcwalton and @nikomatsakis implement the drop changes in the compiler, where drop flags won't be needed at all, assuming that works out

it needs to be updated to zero in the take glue instead of in the drop glue though for correctness