C-style callbacks often rely on `void` pointer arguments that are unsafe
because of the removed type. C++ functors allow passing arbitrary data
along the callback, plus convenient features like defining the callback
using a lambda.
Didn't port the `typedef` because it doesn't add much in this case, just
hides the type from the reader who has to look it up first.
Note that this function isn't used in the main branch currently.
cdd4beeb5e