Since Lua function arguments are not typed, there is no
explicit "const", and a function can always modify the parameter.
When passing `ChanMapping const&` as argument, the object is
copy constructed. In this specific case the std::map<> members
of ChanMapping allocate memory.
Passing a pointer to the object works around this issue.
LuaBridge later dereferences the object as needed when calling
c++ methods, and copy-construction would only happen later.
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