Introduce a new field to `Environment` to store a list of pipeline
generator functions.
Add a `run_pipeline` method to `Environment` that iterates through and
executes all registered generators until they are exhausted.
Implement `ObservableStream` trait for `RootStream` and `PipeStream`.
Add a `StreamNode` wrapper for `ObservableStream` to be used as a script
object.
Register `create-random-ohlc` as a native function that creates a
`RootStream`, sets up a OHLC record layout, and registers a stateful
generator closure in the environment's pipeline generators.
Michael Schimmel