add a shortCircuitSwitch operator

[codemile]

This is an operator that adds caching to an observable stream.

shortCircuitSwitch<T>(key: (v: T) => string | number, ob$: Observable<T>);

When a value is emitted the operator will use the key returned by the callback to see if here is a cached value internally (this might be a passed in reference). If there is no value the observable is switchMap() and the value from that is cached. Otherwise the cached value is emitted.

I'm not sure of all the edge cases for this.

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