feat: add missing furlang/utility/hash.hpp

I forgot to stage it from untracked lol

furlang/include/furlang/utility/hash.hpp

@@ -0,0 +1,47 @@
+#ifndef FURLANG_UTILITY_HASH_HPP
+#define FURLANG_UTILITY_HASH_HPP
+
+#include <cstddef>
+#include <functional>
+
+namespace furlang {
+namespace utility {
+
+// Source - https://stackoverflow.com/a/27952689
+// Posted by Yakk - Adam Nevraumont, modified by community. See post 'Timeline' for change history
+// Retrieved 2026-07-07, License - CC BY-SA 4.0
+static inline std::size_t hash_combine(std::size_t lhs, std::size_t rhs) {
+    if constexpr (sizeof(std::size_t) >= 8) {
+        lhs ^= rhs + 0x517cc1b727220a95 + (lhs << 6) + (lhs >> 2);
+    } else {
+        lhs ^= rhs + 0x9e3779b9 + (lhs << 6) + (lhs >> 2);
+    }
+
+    return lhs;
+}
+
+// Source - https://stackoverflow.com/a/20602159
+// Posted by Casey, modified by community. See post 'Timeline' for change history
+// Retrieved 2026-07-07, License - CC BY-SA 3.0
+template <typename T, typename U, typename FirstHash = std::hash<T>, typename SecondHash = std::hash<U>>
+struct pair_hash {
+    std::size_t operator()(const std::pair<T, U>& pair) const {
+        return hash_combine(FirstHash()(pair.first), SecondHash()(pair.second));
+    }
+};
+
+template <typename T, typename Hash = std::hash<T>>
+struct vector_hash {
+    std::size_t operator()(const std::vector<T>& vec) const {
+        std::size_t seed = 0;
+        for (const auto& element : vec) {
+            hash_combine(seed, Hash()(element));
+        }
+        return seed;
+    }
+};
+
+} // namespace utility
+} // namespace furlang
+
+#endif // FURLANG_UTILITY_HASH_HPP