* [swift] introduce result type as response library
* iOS - run petstore samples
* [swift4] add result sample to testing pipeline
* [swift] update docs with result type
* [swift] update result visibility
* [swift] update docs
* Add queue parameter to API
* Apply queue to response function argument
* Update petstore sample
* Update all sample projects
Run ./bin/swift4-all.sh then remove unrelated diff.
* Refactor variable name
Rename queue to apiResponseQueue, because apiResponseQueue is more clear for explaining the context.
* Convert not string enum default value to string by toString.
* Add some test cases for enum's default value.
* Convert string default value to enum var name format.
* sync master, update samples
