AST Cleanup. Fixes #47 and #41.

Two small cleanups. Hope you don't mind I did them together.
Here's my writing:

```
// regjsparser
//
// ==================================================================
//
// See ECMA-262 Standard: 15.10.1
//
// NOTE: The ECMA-262 standard uses the term "Assertion" for /^/. Here the
//   term "Anchor" is used.
```