message: refactor(mapTo): update function signature
  Reduce the number of generics used in the 'mapTo' function to 1 generic parameter. Mark the 2 generic variant as a deprecated variant
  Fixes ReactiveX/rxjs#5090
diff: @@ -3,6 +3,10 @@ import { Subscriber } from '../Subscriber';
  import { Observable } from '../Observable';
  import { OperatorFunction } from '../types';
  +export function mapTo<R>(value: R): OperatorFunction<any, R>;
  +/** @deprecated remove in v8. Use mapTo<R>(value: R): OperatorFunction<any, R> signature instead **/
  +export function mapTo<T, R>(value: R): OperatorFunction<T, R>;
  +
  /**
  * Emits the given constant value on the output Observable every time the source
  * Observable emits a value.
  @@ -35,8 +39,8 @@ import { OperatorFunction } from '../types';
  * @method mapTo
  * @owner Observable
  */
  -export function mapTo<T, R>(value: R): OperatorFunction<T, R> {
  - return (source: Observable<T>) => source.lift(new MapToOperator(value));
  +export function mapTo<R>(value: R): OperatorFunction<any, R> {
  + return (source: Observable<any>) => source.lift(new MapToOperator(value));
  }
  class MapToOperator<T, R> implements Operator<T, R> {
  