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

test/fixtures/flow/declare-module/invalid-module-in-module/actual.js

@@ -0,0 +1 @@
+declare module A { declare module B {} }

test/fixtures/flow/declare-module/invalid-module-in-module/options.json

@@ -0,0 +1,3 @@
+{
+  "throws": "`declare module` cannot be used inside another `declare module` (1:27)"
+}