`grow_to` method is implemented

Increases the capacity of the vector at least up to the new_capacity:

• has no affect if new_capacity <= self.capacity(), and returns Ok(self.capacity());
• increases the capacity to x >= new_capacity otherwise if the operation succeeds.

This method is unsafe due to the internal guarantees of pinned vectors.

• A SplitVec, on the other hand, can grow to the new_capacity without any problem.
  However, it is not designed to have intermediate empty fragments, while grow_to can leave such fragments.
  Hence, the caller is responsible for handling this.