feat: Add streams module for reactive pipeline

Introduces the streams module, defining core reactive primitives like
Signal,
Stream, Observer, RootStream, and PipeStream. This module lays the
groundwork
for building reactive data processing pipelines.

Includes new types for managing reactive data flow and a basic test
suite to
verify the functionality of RootStream and PipeStream.

src/ast/rtl/mod.rs

@@ -3,6 +3,7 @@ pub mod datetime;
 pub mod intrinsics;
 pub mod math;
 pub mod series;
+pub mod streams;
 pub mod type_registry;
 
 use crate::ast::environment::Environment;