Add Optional type for filter pipes

Introduces `StaticType::Optional` to represent values that can be either
of a specific type `T` or `Void`. This is crucial for handling
situations in filter pipes where an expression might not always produce
a value.

The type checker now correctly deduces and propagates this `Optional`
type, and the pipeline operator (`pipe`) is updated to unwrap
`Optional(T)` results, yielding `T`. This ensures that the type system
accurately reflects the potential absence of values in intermediate
pipeline steps.

Also includes a minor rename in the tuple-struct example from `pipe` to
`p` for clarity, and registers the `streams` module.

src/ast/rtl/mod.rs

@@ -13,4 +13,5 @@ pub fn register(env: &Environment) {
     datetime::register(env);
     math::register(env);
     series::register(env);
+    streams::register(env);
 }