Interconverting `std::function` with `copyable_function`

C++11 introduced std::function as a type-erased callable-object holder. function was badly designed in a few ways: Its rarely used “go fish” API bloats every user; it advertises operator() const even when the controlled object’s operator() is mutating; it handles what should be a precondition violation by throwing an exception, which again bloats every user and means its operator() can never be…