Function eval_let_star 

pub fn eval_let_star(
    cx: &mut Cx,
    outer: &LexicalEnv,
    bindings: Vec<(Symbol, BindingInitializer)>,
    body: impl FnOnce(&mut Cx, &LexicalEnv) -> Result<Value>,
) -> Result<Value>

Evaluates a let* form: sequential bindings in a fresh child scope.

Each BindingInitializer runs in order and sees the bindings established before it.