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/interfaces-module-and-script/5/expected.json

@@ -109,6 +109,8 @@
                 },
                 "name": "length"
               },
+              "static": false,
+              "kind": "init",
               "value": {
                 "type": "NumberTypeAnnotation",
                 "start": 56,
@@ -124,9 +126,8 @@
                   }
                 }
               },
-              "optional": false,
-              "static": false,
-              "variance": null
+              "variance": null,
+              "optional": false
             }
           ],
           "indexers": [