let_op() is no longer the default implementation of op(). Rationale: let_op()

let_op() is no longer the default implementation of op(). Rationale: let_op()
needs to clear status_flags::hash_calculated, otherwise we could end up with
equal objects that have different hash values. But clearing hash_calculated
on a read-only op() access would be inefficient. let_op() is now only
implemented by lst and matrix.