Because they are friendlier to use in C++ code than the existing mesh
mapping API, these mappings from one domain to another were often
reimplemented in separate files. This commit moves some basic
implementations to a `mesh_topology` namespace in the existing
mesh mapping header file. These is plenty of room for performance
improvement here, particularly by not using an array of Vectors, but
that can come later.
Split from D16029
25533dbe21