feat(function_ref): add as_c_callback() member function

function_ref.hpp

@@ -135,6 +135,19 @@ class function_ref<Signature, R(Args...)>
       assert(*this && "*this does not store a callable function target");
       return _erased_func(_invocable, std::forward<Args>(args)...);
     }
+
+    // Helper that exposes the guts of this class to make it easily usable in C-style callbacks.
+    // This avoids extra indirection & conversion overhead.
+    // Bit hackis right now, should probably perform some proper conversions to actually rely on defined behavior...
+    std::pair<void* /* c_cb_arg */, R (*)(void* c_cb_arg, Args...) noexcept(is_noexcept)> as_c_callback() const noexcept {
+      return {
+        _invocable.optr,
+        reinterpret_cast<R (*)(void* c_cb_arg, Args...) noexcept(is_noexcept)>(
+          // casting via intermediate most basic function pointer to suppress a warning about incompatible function pointer type.
+          reinterpret_cast<void (*)() noexcept(is_noexcept)>(_erased_func)
+        ),
+      };
+    }
 };
 
 template <typename R, typename... Args>