`Iterable.whereType` is not available as async version on `Stream.whereType`

[mkustermann]

Most methods on Iterable are available on Stream making it easy for users to think about the ladder as the asynchronous version of the former.

Though it seems that e.g. Iterable.whereType is not available as async version on Stream.whereType.

/cc @lrhn

[lrhn]

Known issue, and the main reason to not just fix it is that it's breaking to add members to the Stream interface.

Streams are not often heterogenous, so you need whereType less often than for Iterable.

Could add it as an extension method. (Most stream methods could just be extension methods, it's extremely rare that anyone overrides a method of Stream, since that means knowing something specific about the events ahead of time.)