Rename AddInstance to AddSingleton

[Eilon]

The name AddInstance doesn't make it super clear what the lifetime is of the object. (If you stop and think about it, it can only be Singleton, but every time I call that method I have to stop and think about it.)

So, we should rename AddInstance to AddSingleton, or perhaps AddSingletonInstance.

[Eilon]

@lodejard @davidfowl @halter73 thoughts on the name?

[lodejard]

Is there an overload where the only difference would be between Type and Object? If that's okay any naming works for me

[pranavkm]

Makes the 3 scenarios line up:

AddSingleton<IService, MyService>();

AddSingleton<IService>(serviceProvider => new MyService(serviceProvider));

AddSingleton<IService>(new MyService());

[halter73]

I'm sold.