Small performance enahancement for merge_options/3.
Small performance enahancement for merge_options/3.

edechter/swipl-devel,jn7163/swipl-devel,edechter/swipl-devel,edechter/swipl-devel,jn7163/swipl-devel,mndrix/swipl-devel,koryonik/swipl-devel,mndrix/swipl-devel,jn7163/swipl-devel,edechter/swipl-devel,mndrix/swipl-devel,jn7163/swipl-devel,koryonik/swipl-devel,mndrix/swipl-devel,edechter/swipl-devel,koryonik/swipl-devel,jn7163/swipl-devel,koryonik/swipl-devel,mndrix/swipl-devel,koryonik/swipl-devel