Update the API to not use the nested classes

[ForNeVeR]

The current architecture with all the classes being nested inside of the giant public static partial class TdApi is sound, but causes some problems: e.g. the IDEs have hard time decompiling it when asked, and also AFAIK there might be some limits on class sizes on Xamarin.

Obviously, this will be a breaking change, but we'll need to think how to make it work properly (at least source-wise) after such an upgrade.