Adding a is_always_lock_free function rather than just failing the compilation

[Lomadriel]

WaitFreeCollections/include/wfc/unordered_map.hpp

Lines 165 to 166 in 5ccd6ab

	static_assert(std::atomic<std::size_t>::is_always_lock_free, "Atomic implementation is not lock free");
	static_assert(std::atomic<node_union>::is_always_lock_free, "Atomic implementation is not lock free");

These two lines makes the compilation crash, maybe it is better to add a bool is_always_lock_free() function since the map still works with lock.