Refactor Purity enum and NativeFunction struct

Move `Purity` enum definition from `optimizer.rs` to `types.rs` and
create a `NativeFunction` struct to hold the function and its purity.
Update `Value::Function` to store `Rc<NativeFunction>` and
`Value::make_function`
helper to simplify creation.

This change allows tracking the purity of native functions, which is
useful
for optimization and static analysis.

src/ast/rtl/datetime.rs

@@ -1,6 +1,5 @@
-use crate::ast::compiler::optimizer::Purity;
 use crate::ast::environment::Environment;
-use crate::ast::types::{Signature, StaticType, Value};
+use crate::ast::types::{Purity, Signature, StaticType, Value};
 use chrono::{NaiveDate, NaiveDateTime};
 
 pub fn register(env: &Environment) {