refactor(furvm): change instruction_t base to furvm::byte

furvm/include/furvm/fwd.hpp

@@ -9,6 +9,8 @@
  */
 namespace furvm {
 
+using byte = std::uint8_t;
+
 // constant.hpp
 
 /**
@@ -36,7 +38,7 @@ class constant;
  * @enum instruction_t
  * @brief Furvm's instruction type.
  */
-enum class instruction_t : std::uint8_t;
+enum class instruction_t : byte;
 
 /**
  * @struct instruction
@@ -44,6 +46,20 @@ enum class instruction_t : std::uint8_t;
  */
 struct instruction;
 
+// function.hpp
+
+/**
+ * @class function
+ * @brief Function.
+ *
+ *
+ * A furvm function.
+ */
+class function {
+public:
+private:
+};
+
 // module.hpp
 
 /**

furvm/include/furvm/instruction.hpp

@@ -5,7 +5,7 @@
 
 namespace furvm {
 
-enum class instruction_t : std::uint8_t {
+enum class instruction_t : byte {
     /**
      * @brief No operation.
      */