Changing API fundamentally to adopt Swift 5 improvements

This version changes AMSMB2's public API to exploit new Swift features, while providing an Objective-C compatibility layer.

• Return Result in completion handler instead of nullable pairs
  • You can use switch to handle result or error, see readme
• Accepting DataProtocol instead of concrete Data
  • You can pass [UInt8] or DispatchData to write(data:) method without casting
• Accepting Range instead of offset-length pair
  • Use the same range you already use to subscript data for contents(atPath:range:) method
• Disconnect gracefully
  • disconnect(graceful: true) will wait until all queued file operations are done and their completion handler is executed.
• Reading from / writing to InputStream/OutputStream directly.
• Fixed reporting incorrect creation date of file