Make ReactiveDerivedCollection schedulable

By default, the scheduler is Scheduler.Immediate, so no change from the current behavior
Autogenerate a WhenAny overload that automatically creates tuples for you
Merge pull request #643 from reactiveui/autogenerate_WhenAny_Tuple

Autogenerate a WhenAnyValue overload that automatically creates tuples for you
Merge pull request #642 from reactiveui/make_derived_collection_schedulable

Make ReactiveDerivedCollection schedulable
Add a ReactiveContentPage<T> as well
Merge branch 'rxui6-master' of github.com:reactiveui/ReactiveUI into rxui6-master
Add a Command method more familiar to the TPL proletariat
Add Cancellation-friendly versions of CreateAsyncTask
Hide poorly designed ICommand methods
Just for you, @onovotny <3
Someday I will be good at Fluency
Add in RxUI 5.x ReactiveCommand for compatibility purposes
These methods are just :trollface:ing, make people use Task.Run
Merge pull request #645 from reactiveui/command-improvements

Finish up ReactiveCommand
Fix WinRT derp
Test scheduling
Get the tests to compile not really RIP me
Fix up WinForms build
Revert "Hide poorly designed ICommand methods"

This reverts commit 19c64700474a747e123d22198dd076cef8e776b3.
Revert "Get the tests to compile not really RIP me"

This reverts commit 33e7895b24ea57f64327de2f1fb796c10059410a.