Update all Transducer reducer signatures to include indexOrKey and fold

[richytong]

Update reducerMap, reducerFilter, reducerFlatMap, reducerForEach, reducerTryCatch

Update all instances of Reducer and Transducer in docs to this

 * type Reducer = (
 *   accumulator any,
 *   value any,
 *   indexOrKey number|string|any,
 *   fold Foldable
 * )=>(nextAccumulator Promise|any)
 *
 * type Transducer = Reducer=>Reducer