refactor externs generation

Summary:
Split the main Annotator class into three pieces:
- the code that does .ts => .ts+Closure annotations,
- the code that does .ts => externs.js,
- and a superclass of the code shared by both.

The class hierarchy (where each level of indent means "subclass")
is now:

Rewriter: generic TypeScript AST rewriter
- ClosureRewriter: Rewriter that understands Closure JSDoc etc.
  - Annotator: ClosureRewriter that does .ts => .ts+Closure
  - ExternsWriter: ClosureRewriter that does .tx => externs

This is almost all just moving code around.  The main function that
changed is Annotator.annotate, to use the new classes.

Should be no functionality change.

Reviewers: rkirov

Reviewed By: rkirov

Subscribers: typescript-eng

Differential Revision: https://reviews.angular.io/D209

Author: evmar

src/rewriter.ts

@@ -42,7 +42,7 @@ export abstract class Rewriter {
    * @return True if the node has been handled and doesn't need to be traversed;
    *    false to have the node written and its children recursively visited.
    */
-  protected abstract maybeProcess(node: ts.Node): boolean;
+  protected maybeProcess(node: ts.Node): boolean { return false; }
 
   /** writeNode writes a ts.Node, calling this.visit() on its children. */
   writeNode(node: ts.Node, skipComments = false) {