ValueStream.map should return ValueStream (?)

[Albert Wolszon]

Hello,

shouldn't ValueStream<T>.map(U Function(T)) return ValueStream<U> instead of Stream<U>?

I'd like to have an easy way to map the value stream while preserving the value stream characteristics.

This is what I used instead: https://gist.github.com/Albert221/c0da244cec07f7f2de317d19d9a56a27

Probably related to #625.