language: introduce the requirement to use return to return a value from a function

bytecode/src/main/kotlin/gay/pizza/pork/bytecode/Op.kt

@@ -3,4 +3,12 @@ package gay.pizza.pork.bytecode
 import kotlinx.serialization.Serializable
 
 @Serializable
-data class Op(val code: Opcode, val args: List<UInt>)
+class Op(val code: Opcode, val args: List<UInt>) {
+  override fun toString(): String = buildString {
+    append(code.name)
+    if (args.isNotEmpty()) {
+      append(" ")
+      append(args.joinToString(" "))
+    }
+  }
+}