The ustring is not a trivially copyable object from the C++ standard
point of view, so using memcpy on it is strictly wrong. In practice,
however, this is OK since it is just a thin wrapper around char*.
For now use explicit cast to void* same as it was done in other places
of ccl::array implementation. But also localize the place where memory
copy happens to make it easier to support proper non-trivial C++
objects in the future.
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