`kind` field for Literal 

It is very useful for basic type checking, and IMO is a bit less hackish (and easier) than `typeof node.value`. Here's my idea: 

``` js
interface Literal <: Node, Expression {
  type: "Literal";
  value: string | boolean | null | number | RegExp;
  // addition, the type
  kind: "string" |
    "boolean" |
    "null" |
    "undefined" |
    "number" |
    "regexp";
}
```
