Function eval

pub(crate) fn eval<'ob>(
    form: &Rt<Slot<Gc<ObjectType<'_>>>>,
    lexical: Option<&Rt<Slot<Gc<ObjectType<'_>>>>>,
    env: &mut Rt<Env<'_>>,
    cx: &'ob mut Context<'_>,
) -> Result<Gc<ObjectType<'ob>>, Error>