refactor(Subscriber): update type declaration for Subscriber

[kwonoj]

• Subscriber applies generic type constraint

Subscriber itself does have type constraint via generic, it actually doesn't work for values or creating new observables, like below code snippet

Rx.Observable.create((observer: Rx.Subscriber<number>) => {
    observer.next(42);
    observer.next('a');
    observer.complete();

    return () => {
        console.log('disposed');
    };
})

should be caught by compiler since type constraint of given subscriber is number, but it doesn't by Subscriber::next() does not apply type constraint for values.

type constraint is not applied for values

PR makes Subscriber now follows given types more explicitly let compiler allows type checking for build time.

type constraint applied after changes

This change also brings correct generic type checking for operators either operator passes same type or either different type like <in, out>. No functional changes are introduced, change affects only for type declaration.

[luisgabriel]

👍

[benlesh]

Merged with 0329b50... thanks, as always @kwonoj!