Introduce Rtl struct and from_rtl constructor

This commit refactors the `Environment` struct to better manage the Rust
Runtime Library (RTL) bootstrap state.

A new `Rtl` struct is introduced to hold a frozen snapshot of the
initial RTL state, including scopes, types, purity, values, and
documentation. This snapshot is created once after `rtl::register()`
completes.

The `Environment::new()` constructor is updated to perform a two-phase
bootstrap: first, it registers RTL functions, and then it freezes the
scope 0 and captures the RTL state into the new `Rtl` struct.

A new constructor, `Environment::from_rtl()`, is added. This allows
creating independent execution environments that all share the same
underlying RTL state. This is crucial for isolation between different
execution contexts, as user definitions and loaded modules will not leak
between environments created from the same `Rtl` snapshot.

The `Environment::list_bindings` and `Environment::list_rtl_docs`
methods are updated to use the `rtl` field for accessing the RTL scope
and documentation, reinforcing the concept of a shared, immutable RTL
state.

A new test, `test_environments_from_shared_rtl_are_independent`, is
added to verify that environments created from the same `Rtl` snapshot
are indeed independent and do not leak user-defined bindings.

tests/rtl.rs

@@ -118,3 +118,22 @@ fn test_date_parsing() {
         panic!("Expected DateTime, got {:?}", res);
     }
 }
+
+#[test]
+fn test_environments_from_shared_rtl_are_independent() {
+    let rtl = Environment::new().rtl();
+
+    // Two fresh environments from the same frozen RTL snapshot.
+    let env1 = Environment::from_rtl(rtl.clone());
+    let env2 = Environment::from_rtl(rtl.clone());
+
+    // Define a variable in env1 only.
+    env1.run_script("(do (def x 42) 0)").unwrap();
+
+    // x must not be visible in env2.
+    assert!(env2.run_script("x").is_err(), "User binding leaked across environments");
+
+    // RTL functions must work in both environments.
+    assert_eq!(format!("{}", env1.run_script("(+ 1 2)").unwrap()), "3");
+    assert_eq!(format!("{}", env2.run_script("(+ 1 2)").unwrap()), "3");
+}