* [through2] Fix typos
* [through2] Change return types to stream.Transform
`through2` creates `stream.Transform`-flavored streams, so switching the return types to `stream.Transform` gives us richer typing information than using `NodeJS.ReadWriteStream`, which is missing methods (e.g. `stream.Transform#push`) and the ability to use object mode.
* [through2] Add through2.ctor method
* [through2] Update header and docs
