HTTP status value naming normalization

There are many place in Swift code where used hard coded values, such as response statuses (200, 201, 404, ...) which can replaced with constants HTTP_OK, HTTP_CREATED, HTTP_NOT_FOUND. Also there is widly used idiom 200 <= status < 300, that can be replaced as well with something like this is_success(status). I want add modules for defining all required constants (Swift, HTTP).

So I think this changes will improve Swift code readability.

PS: this is an initial changes in github https://github.com/vitoordaz/swift/commit/7163d5df13ceaf8fc7b53ba812fe16bd7dd31131