Convert the various Mapping::get_*_data() functions to return std::unique_ptr.

Convert the various Mapping::get_*_data() functions to return std::unique_ptr.

This required a number of adjustments in places where we used the fact that we used covariant
return types all over the place. We can't do this any more, and so in a number of places
we have to convert things by hand back to the (known) derived type.