Optimize generated Swift API

[asutula]

Parts of the API feel a bit strange when used from Swift. Obj C provides annotations that give us lots of control over how things will look from Swift. Need to dig in there and make some improvements.

Here's a list of potential improvements:

• [ ] Methods should throw exceptions instead of take error pointer parameters
• [ ] Textile.initialize should return an optional string. Currently, when the underlying return value is actually null, it probably causes a crash in Swift because the Swift return type is non-optional String
• [ ] General correct usage of optional and non-optional return values
• [ ] Swift doesn't resolve some data types like ThreadList and Thread, they show as Any

[asutula]

Just throwing some references in here that can be used when I fix this:

• https://developer.apple.com/documentation/swift/objective-c_and_c_code_customization
• https://stackoverflow.com/questions/34786115/swift-doesnt-convert-objective-c-nserror-to-throws
• https://developer.apple.com/documentation/swift/cocoa_design_patterns/about_imported_cocoa_error_parameters