Avoid var special in core macros for private var access

There are two places where macros in cljs.core were revised to use var in order to access private vars in cljs.core without triggering the private var warning introduced with CLJS-1702.

In order to avoid the potential emitted code bloat that might occur (these var uses are in the defrecord and defaulti macros), just use JavaScript interop to directly access the var values in question.