Reverted semantics of assign_by.

The problem described in previous commit was caused only inside the
cell's copy constructor, where an assignemt for `std::unique_ptr` was
attempted. Since `std::unique_ptr`'s cannot be copy-assigned, I have
made that assignment manually moving the just cloned other's segment to
the cell being constructed. The original `assign_by` method, relying on
STL's `assign()` cannot accommodate this without changing its
semantics (as pointed out by @halfflat), which would then also conflict
with STL's semantics.