Add support for declare export + fixes (#224)

* Add support for declare export

* Do not allow declare module inside declare module

* Reallow module exports outside module

* Add handling of `declare export default`

Add check for multiple module.export declarations

* Disallow export let/const/type

Refactor parsing object properties to share more code and add support
for getters and setters

* Rename tests

* Update test fixtures

* Optimize for performance

* disallow declare export interface outside of module

* Refactor code to be more readable and less lookahead

* Add comments

* Add test for export star as

* Test for number literal getter/setter

* Add more tests

* Fix tests

* Allow union types and correctly eat semi after type

* Use non computed keys

* Fix tests

src/parser/statement.js

@@ -810,7 +810,8 @@ pp.parseClassSuper = function (node) {
 // Parses module export declaration.
 
 pp.parseExport = function (node) {
-  this.next();
+  this.eat(tt._export);
+
   // export * from '...'
   if (this.match(tt.star)) {
     const specifier = this.startNode();