Adding various features for stream processing

[Alex Klymenko]

Need to add next set of features to library:

filterByType: Filters a stream by a specific class type.
toList: Collects the elements of a stream into a List.
toSet: Collects the elements of a stream into a Set.
toMap: Collects the elements of a stream into a Map.
groupBy: Groups the elements of a stream by a classifier function.
partitionBy: Partitions the elements of a stream by a predicate.
streamOfNullable: Creates a stream from a nullable element.
concatStreams: Concatenates multiple streams into one.
zip: Zips two streams into one.
peekAndReturn: Applies an action to each element of the stream and returns the same element.
findFirst: Returns the first element of the stream, if any.
batchProcess: Processes elements of the stream in batches.
parallelFilter: Filters a stream in parallel.
parallelMap: Maps elements of a stream in parallel.
distinctByKey: Ensures distinct elements based on a key extractor function.
streamify: Converts an Iterator or Iterable into a Stream.

also change documentation and tests