Introduce traverse(), improve map() perf

There was a perf regression in map() in
889d8d2b99d982c9660fab662915ca14fcb1fdfa. This change not only fixes
the regression, but improves map() perf well beyond what it was
previously by using traverse(), which is an abstracted version of the
very fast all() algorithm.  Now, both all() and map() use traverse().
Merge pull request #390 from cujojs/improve-map-perf

Improve map() perf