Modernize to C++17: replace deprecated aligned_storage, add nodiscard

[HFTrader]

Summary

Bump minimum standard from C++11 to C++17 and apply modernizations:

• Replace std::aligned_storage (deprecated C++23, removed C++26) with alignas + std::byte arrays in bucket_container. Use std::launder for pointer access to constructed objects, and a separate storage_ptr() for construct/destroy on uninitialized storage
• Add [[nodiscard]] to public query functions: hash_function(), key_eq(), get_allocator(), hashpower(), bucket_count(), empty(), size(), capacity(), load_factor(), contains()
• Mark primary constructor explicit to prevent implicit conversion from size_type
• Update CMake target_compile_features from cxx_constexpr to cxx_std_17

Test plan

• All existing unit tests pass
• Universal benchmark builds and runs
• Verified with GCC 13.3

[carlocorradini]

This is super useful since my compiler screams on every insert because of std::aligned_storage 😥

[carlocorradini]

@HFTrader Instead of forcing C++17, add a check to use alignas + std::byte instead of std::aligned_storage.
Do the same for [[nodiscard]] and other features that are only available from specific C++ versions.

I don't think they'll merge this, since forcing C++17 would be a breaking change.
It would be better to keep compatibility checks and use newer features only when the target C++ version supports them.

[HFTrader]

@carlocorradini thanks for the feedback — and good point that a hard bump is the heavier ask of the two.

A couple of things worth weighing, though:

1. std::aligned_storage is deprecated in C++23 (P1413), which is why your compiler is screaming. So the long-term direction is alignas + std::byte regardless — the question is just whether libcuckoo carries both code paths or picks one.
2. [[nodiscard]] can be gated cleanly with __has_cpp_attribute(nodiscard) (or a LIBCUCKOO_NODISCARD macro), so that one is cheap to make conditional. Happy to do that.
3. The std::aligned_storage → alignas(T) std::byte[N][sizeof(T)] swap is also doable under C++11 — std::byte is C++17, but alignas + unsigned char works in C++11 and is functionally equivalent. The std::launder calls would need a __cpp_lib_launder guard (it's a no-op in practice pre-C++17, since the old reinterpret_cast path was what everyone relied on).

That said: the repo's last release was 2019 and there's a backlog of unmerged PRs, so I'm not optimistic either way. If a maintainer chimes in preferring the gated approach, I'm glad to rework it that way — just didn't want to add #if __cplusplus >= … scaffolding speculatively.