chore: flat out the file structure

include/furvm/detail/handle.hpp

@@ -0,0 +1,39 @@
+#ifndef FURVM_DETAIL_HANDLE_HPP
+#define FURVM_DETAIL_HANDLE_HPP
+
+#include <functional>
+#include <type_traits>
+
+namespace furvm {
+namespace detail {
+
+/**
+ * @brief Default specialization for header_has_refcount type trait.
+ */
+template <typename Header, typename = void>
+struct header_has_refcount : std::false_type {};
+
+/**
+ * @brief Specialization for header_has_refcount type trait.
+ */
+template <typename Header>
+struct header_has_refcount<Header,
+    std::void_t<decltype(std::declval<Header&>().acquire()),
+        decltype(std::declval<Header&>().release()),
+        decltype(std::declval<Header&>().reference_count())>> : std::true_type {};
+
+/**
+ * @brief An alias for header_has_refcount's value.
+ */
+template <typename Header>
+static constexpr auto header_has_refcount_v = header_has_refcount<Header>::value;
+
+template <typename Handle, typename IdHash = std::hash<typename Handle::id_type>>
+struct handle_hash {
+    std::size_t operator()(const Handle& handle) const { return IdHash{}(handle.id()); }
+};
+
+} // namespace detail
+} // namespace furvm
+
+#endif // FURVM_DETAIL_HANDLE_HPP