[FEATURE REQUEST] Extension function on Stream

[lohnn]

Is your feature request related to a problem? Please describe.
Now that we have added isolate stream functionality I want to make it easier to run functions in an isolate when building up a call chain for a stream.

Describe the solution you'd like
An extension function on Stream that allows for mapping a value asynchronously by running the function in an isolate.
Example: Stream.isolateMap(Function) // running in an isolate

Additional context
Questions that needs to be answered for implementation of this:

• What isolate should the function be run in? (Maybe from the new isolate pool, or a way to select which pool from [FEATURE REQUEST] Add isolate pools (thread pools) #25?)