get_macro(): Use count() and at() instead of iterating over complete map
get_macro(): Use count() and at() instead of iterating over complete map

This reduces the complexity of the function from linear (in the number of macros) to
logarithmic.

newsboat/newsboat,newsboat/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat