Explicitly forget the zero remaining elements in `vec::IntoIter::fold()` and `spec_extend()`.

Adds internal `vec::IntoIter::forget_remaining_elements_and_dealloc()`,
which is used by `fold()` and `spec_extend()`, when those operations
complete, to forget the zero remaining elements and only deallocate the
allocation, ensuring that there will never be a useless loop to drop
zero remaining elements when the iterator is dropped.