It's long been a guideline (and IIRC a Weff-c++ warning) that either all, or
none, of the copy methods should be defined, but this became a standard warning
in GCC9. Presumably to account for a later language change though I'm not sure
which.
I don't remember why the ChanMapping copy constructor can't just be a simple
copy (it's just a map of POD), but figure it's safer to just copy what that
does.
