Use a specialized walker for optional choices.
Use a specialized walker for optional choices.

RNG simplification turns ``optional`` patterns into patterns of the form
``<choice><empty/>p</choice>`` these can benefit from a specialized walker that
optimizes the logic.

lddubeau/salve,mangalam-research/salve,lddubeau/salve,mangalam-research/salve,mangalam-research/salve,lddubeau/salve