feat(furvm): add call instruction

furvm/include/furvm/function.hpp

@@ -14,7 +14,7 @@ enum class function_t : std::uint8_t {
     Native,     /**< A native function implemented through furvm API. */
 };
 
-using native_function = std::function<void(const executor_p&)>;
+using native_function = std::function<void(executor&)>;
 
 class function {
 private:

furvm/include/furvm/instruction.hpp

@@ -65,6 +65,13 @@ enum class instruction_t : byte {
      */
     Mod,
 
+    /**
+     * @brief Calls a function.
+     *
+     * Calls a function denoted by next two bytes in little-endian from current frame's module.
+     */
+    Call,
+
     /**
      * @brief Pops the current call frame.
      */

furvm/include/furvm/module.hpp

@@ -51,6 +51,14 @@ public:
      * @return The byte.
      */
     constexpr byte byte(std::size_t offset) const { return m_bytecode.at(offset); }
+public:
+    /**
+     * @brief Returns a function from this module.
+     *
+     * @param id Id of the function.
+     * @return The function.
+     */
+    constexpr const function_p& function_at(function_handle id) const { return m_functions.at(id); }
 private:
     module_handle           m_id;
     bytecode_t              m_bytecode;