map-invert should use transients and reduce-kv instead of reduce

Two performance enhancements on clojure.set/map-invert:

1) Use reduce-kv to avoid realizing mapentry's from input map
2) Use transients to create the output map

Perf:

Patch: clj-1808-map-invert-should-use-reduce-kv-and-transient.patch

Screened by: Alex Miller