move cljs.core/exists? hack, more AOT friendly
move cljs.core/exists? hack, more AOT friendly

somehow the in-ns/defmacro variant doesn't apply in AOT releases,
leading to the cljs.core/exists? variant being used in some
uncommon edge cases.

fixes #839

thheller/shadow-devtools,thheller/shadow-devtools,thheller/shadow-cljs,thheller/shadow-devtools,thheller/shadow-cljs,thheller/shadow-cljs,thheller/shadow-devtools,thheller/shadow-cljs