Introduce closure AST node

The `Closure` struct has been moved from `ast/vm.rs` to a new module
`ast/closure.rs`. This improves the organization of the AST nodes. The
`Closure` struct represents a compiled function body along with its
captured upvalues, which is a key feature for Myc Script.

src/ast/vm.rs

@@ -1,58 +1,11 @@
-use crate::ast::nodes::{Address, AnalyzedNode, ExecNode, IdentifierBinding, NodeKind, StackOffset};
+use crate::ast::closure::Closure;
+use crate::ast::nodes::{Address, ExecNode, IdentifierBinding, NodeKind, StackOffset};
 use crate::ast::rtl::series::{RecordSeries, SeriesView};
 use crate::ast::rtl::streams::{build_map_stream, build_pipeline_node, StreamNode};
 use crate::ast::types::{Object, PipeFn, Value};
-use std::any::Any;
 use std::cell::RefCell;
 use std::rc::Rc;
 
-#[derive(Debug, Clone)]
-pub struct Closure {
-    /// The executable parameter pattern.
-    pub parameter_node: Rc<ExecNode>,
-    /// The analyzed body (before TCO).
-    pub function_node: Rc<AnalyzedNode>,
-    /// The executable node (after TCO).
-    pub exec_node: Rc<ExecNode>,
-    pub upvalues: Vec<Rc<RefCell<Value>>>,
-    pub positional_count: Option<u32>,
-    /// The number of stack slots required for this closure (calculated late).
-    pub stack_size: u32,
-}
-
-impl Closure {
-    #[inline]
-    pub fn new(
-        params: Rc<ExecNode>,
-        body: Rc<AnalyzedNode>,
-        exec: Rc<ExecNode>,
-        upvalues: Vec<Rc<RefCell<Value>>>,
-        positional_count: Option<u32>,
-        stack_size: u32,
-    ) -> Self {
-        Self {
-            parameter_node: params,
-            function_node: body,
-            exec_node: exec,
-            upvalues,
-            positional_count,
-            stack_size,
-        }
-    }
-}
-
-impl Object for Closure {
-    fn type_name(&self) -> &'static str {
-        "closure"
-    }
-    fn as_any(&self) -> &dyn Any {
-        self
-    }
-    fn into_rc_any(self: Rc<Self>) -> Rc<dyn Any> {
-        self
-    }
-}
-
 #[derive(Debug)]
 struct CallFrame {
     stack_base: usize,