Extra .tupleof field in structs with disabled postblit blocks non-GC-allocation trait

[Per Nordlöw]

On Wednesday, 9 May 2018 at 14:20:41 UTC, Per Nordlöw wrote:
> If so, we can temporarily modify the trait to exclude the last 
> `void*` member of the `S.tuple`. Given that it's always added 
> as the last member.

Note that `std.traits.isCopyable!S` cannot be used, because it 
will return true when `S` has uncopyable members regardless of 
whether S.tupleof have any extra void* element or not (because of 
S's disabled postblit).