Add the `keyed` macro for avoiding repetition in map literals

Useful (ha!) when you've created locals with the same name as the
keys.
Move keyword-map to plumbing.map, add a test.
Fix CLJS tests

I'd forgotten to add special macro handling for cljs.
Merge pull request #46 from gfredericks/keyed

Add the `keyed` macro for avoiding repetition in map literals