Codegen for enum types

This generates a flow type for enums:

```noms
enum Handedness {
  right
  left
  switch
}
```

Generates

```js
type E =
  0 |  // right
  1 |  // left
  2;  // switch
```

Issue #1081

nomdl/codegen/codegen.go

@@ -280,6 +280,7 @@ func (gen *codeGen) readTemplates() *template.Template {
 			"valueToDef":    gen.generator.ValueToDef,
 			"valueToUser":   gen.generator.ValueToUser,
 			"valueZero":     gen.generator.ValueZero,
+			"isLast":        gen.generator.IsLast,
 		}).ParseGlob(glob))
 }