feat(furvm): introduce module, instruction and constant

furvm/include/furvm/instruction.hpp

@@ -0,0 +1,45 @@
+#ifndef FURVM_INSTRUCTION_HPP
+#define FURVM_INSTRUCTION_HPP
+
+#include "furvm/fwd.hpp"
+
+namespace furvm {
+
+enum class instruction_t : std::uint8_t {
+    /**
+     * @brief No operation.
+     */
+    NoOperation = 0,
+
+    /**
+     * @brief Pushes a constant onto the stack.
+     *
+     * Pushes a constant from the constant pool denoted by two next bytes in low-endian onto the stack.
+     */
+    PushConstant,
+
+    /**
+     * @brief Pops top element from the stack.
+     */
+    Drop,
+
+    /**
+     * @brief Duplicates top element on the stack.
+     */
+    Duplicate,
+};
+
+struct instruction {
+    instruction_t type; /**< Instruction type. */
+
+    /**
+     * @brief Instruction value.
+     */
+    union value {
+        constant_index constant; /**< Constant instruction argument. */
+    } value;                     /**< Instruction value. */
+};
+
+} // namespace furvm
+
+#endif // FURVM_INSTRUCTION_HPP