CLJS-1034: Support REPL-defined functions in stacktrace infrastructure
CLJS-1034: Support REPL-defined functions in stacktrace infrastructure

Make `cljs.repl/mapped-stacktrace` nil safe for all the stack element
components.

If handed :file that starts with "<" or nil :file prepend "NO_SOURCE_FILE"
a la Clojure.

Smarter printing for nil values of a stack element.

mstang/clojurescript,mstang/clojurescript,mstang/clojurescript