Work around gcc warning about [[noreturn]] main()

[z33ky]

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[z33ky]

enum class is not implicitly castable to int, so we'd have to use return static_cast<int>(exit_code::SUCCESS);, which is a handful. I'm open to it though, especially because we don't need to write it often. We could maybe also define a short-hand, like exit_cast, though it doesn't save much.

Destructors not running could be bad. We could also switch to a plain enum, but we'd loose the scoping, which I'm quite fond of. We could work around it with a non-constructible struct or namespace with constexpr members. Anyways, discussion in #53.