Enable DCE by avoiding ^:export for cljs use.

[rauhs]

The ^:export declaration are only necessary for the javascript build of datascript. But this eliminates any dead code elimination by the Google Closure compiler which would be nice for people not using certain features. For instance, I don't use FilteredDB.

By making the javascript build a little more involved, we could remove those export declaration for cljs use. cljsee can be used to process the datascript files:

(cljsee.parse/process-source-code
  ;; The first tag will be output if it's in the features list:
 "#?(
  :js   (def ^:export datom? db/datom?)
  :cljs (def datom? db/datom?)
 )"
 #{:cljs :js})

This could allow even more fine grained code generation. Not sure if the trouble is worth it though.

A good enough solution would be to just change the code in datascript.core to:

(goog-define datascript-export-js-api true)

(def datom db/datom)
(when ^boolean datascript-export-js-api
  (goog/exportSymbol "datom" datom))

And allow cljs-only folks to define datascript-export-js-api to false.

Thoughts?