Add series, SMA, and WMA indicators

Introduce new indicators for Simple Moving Average (SMA) and Weighted
Moving Average (WMA), along with their Hull Moving Average (HMA)
derivative.

Also refactors series implementation to use `RefCell` for interior
mutability and adds `SeriesMember` trait for better dynamic series
access. Updates `soa_series.myc` example to reflect new series creation
and push syntax.

src/ast/vm.rs

@@ -460,7 +460,7 @@ impl VM {
                 Ok(Value::Object(Rc::new(closure)))
             }
             BoundKind::Call { callee, args } => {
-                let mut func_val = self.eval_internal(obs, callee)?;
+                let func_val = self.eval_internal(obs, callee)?;
 
                 let base = self.stack.len();
                 if let Err(e) = self.eval_args_to_stack(obs, args) {