Suggestion: make let return any

[staeke]

Suggestion: change let signature (same implementation) from

function letProto<T, R>(this: Observable<T>, func: (selector: Observable<T>) => Observable<R>): Observable<R> ;

to

function letProto<T, R>(this: Observable<T>, func: (selector: Observable<T>) => R): R;

It seems to me that returning an Observable is an unnecessary constraint. Generalizing let would make it usable for returning other custom types taking observables as parameters and doing whatever with them.