Add pipeline generator registration

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.

src/ast/environment.rs

@@ -33,6 +33,7 @@ pub struct Environment {
     pub debug_mode: bool,
     pub optimization: bool,
     pub macro_registry: Rc<RefCell<MacroRegistry>>,
+    pub pipeline_generators: Rc<RefCell<Vec<Box<dyn FnMut() -> bool>>>>,
 }
 
 struct EnvFunctionRegistry {
@@ -106,6 +107,7 @@ impl Environment {
             debug_mode: false,
             optimization: true,
             macro_registry: Rc::new(RefCell::new(MacroRegistry::new())),
+            pipeline_generators: Rc::new(RefCell::new(Vec::new())),
         };
         env.register_stdlib();
         env
@@ -115,6 +117,20 @@ impl Environment {
         self.debug_mode = enabled;
     }
 
+    /// Pumps data through all registered pipeline generators until they are exhausted.
+    pub fn run_pipeline(&self) {
+        let mut generators = self.pipeline_generators.borrow_mut();
+        let mut any_active = true;
+        while any_active {
+            any_active = false;
+            for generator in generators.iter_mut() {
+                if generator() {
+                    any_active = true;
+                }
+            }
+        }
+    }
+
     fn get_expander(&self) -> MacroExpander<RuntimeMacroEvaluator> {
         let evaluator = RuntimeMacroEvaluator {
             global_names: self.global_names.clone(),