Refactor: Use root_purity instead of global_purity

This commit refactors the purity analysis from using a
`HashMap<GlobalIdx, Purity>` to a `Vec<Purity>` indexed by
`GlobalIdx.0`.

This change simplifies the data structure and makes purity lookups more
efficient. The `Binder`'s `globals` field has also been removed as it
was not being used.

src/ast/rtl/streams.rs

@@ -413,7 +413,7 @@ pub fn register(env: &Environment) {
 
     // (create-ticker condition-closure) -> StreamNode
     let ticker_generators = env.pipeline_generators.clone();
-    let globals = env.global_values.clone();
+    let globals = env.root_values.clone();
 
     env.register_native_fn(
         "create-ticker",