Activator performance improvement

Shouldn't close the previous PR early. Turns out I did not have to change much of my design.

Pinging previous trackers
@pranavkm
@halter73 
@rynowak
@ipjohnson
@yishaigalatzer 
I do not understand why we need a new IService for this. Why not just register our own TypeActivator as a normal singleton service? Since we are providing an instance anyway, the call to `Create` would be just as fast.

Do we really want to support using a custom ITypeActivator for activating services in our custom ServiceProvider? I thought the intention was that our ServiceProvider would only have the bare minimum functionality needed by our frameworks. If you want any extensibility, you should use a real IoC container. I really don't see the need for adding any complexity to support bootstrapping a custom ITypeActivator.

This isn't to say that it shouldn't be possible to supply a custom ITypeActivator to be used by our frameworks (e.g. MVC), I just don't think we need to complicate our ServiceProvider code to use it.