MOBSDK-656: Code modernisation: XCUnit and 64-bit support

Includes:
- MOBSDK-657: ObjectiveCMIS: Convert to modern Objective-C code
- MOBSDK-659: ObjectiveCMIS: Ensure 64-bit compatibility
- MOBSDK-664: ObjectiveCMIS: Update testing framework to XCUnit
- MOBSDK-660: SDK: Convert to modern Objective-C code
- MOBSDK-662: SDK: Ensure 64-bit compatibility
- MOBSDK-663: SDK: Update unit testing framework to XCUnit