Elaborator reflection for generating `isXXX` functions for enum types

This feature is inspired by @roife.

For such enum type:

```ts
type Foo = [A | B];
```

It's better that we could have some reflection techniques to generate these functions:

```ts
function isA<'a>(a: [A | 'a]): boolean;
function isB<'a>(a: [B | 'a]): boolean;
```