feat: add return type to function

Closes: #51

furvm/include/furvm/executor.hpp

@@ -5,6 +5,7 @@
 #include "furvm/module.hpp" // IWYU pragma: keep
 #include "furvm/thing.hpp"  // IWYU pragma: keep
 
+#include <optional>
 #include <stack>
 #include <utility>
 #include <vector>
@@ -40,7 +41,8 @@ public:
         std::size_t position;  /**< Cursor to a current instruction in the bytecode. */
         std::size_t stackBase; /**< Snapshot of the stack size before this frame. */
 
-        std::vector<thing_h> variables; /**< Frame variables. */
+        thing_type*          returnType; /**< Return type. */
+        std::vector<thing_h> variables;  /**< Frame variables. */
     };
 public:
     /**

furvm/include/furvm/function.hpp

@@ -6,6 +6,7 @@
 #include "furvm/handle.hpp" // IWYU pragma: keep
 
 #include <cstdint>
+#include <optional>
 #include <stdexcept>
 #include <string>
 #include <type_traits>
@@ -37,7 +38,8 @@ struct import_function {
  * @brief Function signature.
  */
 struct function_sig {
-    std::vector<mod_type_h> params;
+    std::vector<mod_type_h>   params;
+    std::optional<mod_type_h> returnType;
 
     bool operator==(const function_sig& rhs) const { return params == rhs.params; }