feat(observable): move subscription to own file

src/asynciterable/toobservable.ts

@@ -1,4 +1,5 @@
-import { Observable, Observer, Subscription } from '../observer';
+import { Observable, Observer } from '../observer';
+import { Subscription } from '../subscription';
 
 class BooleanSubscription implements Subscription {
   public isUnsubscribed: boolean = false;

src/observer.ts

@@ -1,3 +1,5 @@
+import { Subscription } from './subscription';
+
 export interface NextObserver<T> {
   next: (value: T) => void;
   error?: (err: any) => void;
@@ -44,10 +46,6 @@ export type PartialAsyncObserver<T> =
   ErrorAsyncObserver<T> |
   CompletionAsyncObserver<T>;
 
-export interface Subscription {
-  unsubscribe: () => void;
-}
-
 export interface Observer<T> {
   closed?: boolean;
   next: (value: T) => void;

src/subscription.ts

@@ -0,0 +1,3 @@
+export interface Subscription {
+  unsubscribe: () => void;
+}