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

compiler/src/main/kotlin/gay/pizza/pork/compiler/StubOpEmitter.kt

@@ -30,6 +30,7 @@ class StubOpEmitter(val compiler: Compiler, val symbol: CompilableSymbol) : Func
 
   fun exit() {
     code.localState.popScope()
+    code.emit(Opcode.None)
     code.emit(Opcode.Return)
   }
 
@@ -211,6 +212,11 @@ class StubOpEmitter(val compiler: Compiler, val symbol: CompilableSymbol) : Func
     }
   }
 
+  override fun visitReturn(node: Return) {
+    node.value.visit(this)
+    code.emit(Opcode.Return)
+  }
+
   override fun visitSetAssignment(node: SetAssignment) {
     val stubVarOrCall = code.localState.resolve(node.symbol)
     if (stubVarOrCall.stubVar == null) {