fix(toObservableValue): accommodate all observable inputs

[benlesh]

• Switches toObservableValue to just use rxjs from. This will ensure it works with all types that rxjs is compatable with. Also, the checks in toObservableValue were redundant if from was used. For example, if x is Observable from rxjs, then from(x) === x would be true. from does all of the necessary checks. Also, from will do the work of figuring out of it happens to be an observable that has interop with RxJS, or is an observable from another instance of rxjs (in node_modules, Node scenario).
• Also x == null will only return true if x is undefined or null.
• Fixes the return types of toObservableValue to be more true to form. If you return
  Observable<X> | Observable<Y> | Observable<Z>, it's really no different than Observable<X | Y | Z>, because there's no static/synchronous way to narrow the type of the Observable to say, Observable<X> in either scenario.
• Removes unecessary types in favor of more obvious explicit types
• Removes superfluous isObservable-type assert function
• Removes unused assertion methods. Not sure what those were used for, as they were exported, but two
  were ill-advised, as they were doing type-narrowing by iterating over an array, making it an O(n) operation just to narrow a type.

[ngrxbot]

Preview docs changes for 7494cef at https://previews.ngrx.io/pr2471-7494cef/