RawVecInner: add missing `unsafe` to unsafe fns

- RawVecInner::grow_exact causes UB if called with len and additional
  arguments such that len + additional is less than the current
  capacity.  Indeed, in that case it calls Allocator::grow with a
  new_layout that is smaller than old_layout, which violates a safety
  precondition.

- All RawVecInner methods for resizing the buffer cause UB
  if called with an elem_layout different from the one used to initially
  allocate the buffer, because in that case Allocator::grow/shrink is called with
  an old_layout that does not fit the allocated block, which violates a
  safety precondition.

- RawVecInner::current_memory might cause UB if called with an elem_layout
  different from the one used to initially allocate the buffer, because
  the unchecked_mul might overflow.

- Furthermore, these methods cause UB if called with an elem_layout
  where the size is not a multiple of the alignment. This is because
  Layout::repeat is used (in layout_array) to compute the allocation's
  layout when allocating, which includes padding to ensure alignment of
  array elements, but simple multiplication is used (in current_memory) to
  compute the old allocation's layout when resizing or deallocating, which
  would cause the layout used to resize or deallocate to not fit the
  allocated block, which violates a safety precondition.

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