AsyncRx.NET Distinct and DistinctUntilChanged should be extension methods

[idg10]

Bug

The AsyncObservable class's static Distinct and DistinctUntilChanged methods are not extension methods. This means that given some:

IAsyncObservable<int> xs = GetNums();

you can't write this:

xs.Distinct(); // or DistinctUntilChanged

You have to write this:

AsyncObservable.Distinct(xs);

This seems like a straightforward oversight. We just need to add the this keyword to these methods.