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

vm/src/main/kotlin/gay/pizza/pork/vm/VirtualMachine.kt

@@ -12,6 +12,8 @@ class VirtualMachine(world: CompiledWorld) : ExecutionContext {
     TrueOpHandler,
     FalseOpHandler,
 
+    NoneOpHandler,
+
     ListMakeOpHandler,
     ListSizeOpHandler,
 

vm/src/main/kotlin/gay/pizza/pork/vm/ops/NoneOpHandler.kt

@@ -0,0 +1,12 @@
+package gay.pizza.pork.vm.ops
+
+import gay.pizza.pork.bytecode.Op
+import gay.pizza.pork.bytecode.Opcode
+import gay.pizza.pork.vm.InternalMachine
+import gay.pizza.pork.vm.OpHandler
+
+object NoneOpHandler : OpHandler(Opcode.None) {
+  override fun handle(machine: InternalMachine, op: Op) {
+    machine.push(Unit)
+  }
+}