CLJS-454: Allow nanoseconds in fractions part of instant literals

This patch mimics the clojure reader which truncates the fraction part to 9 digits (i.e. nanoseconds). This patch truncates to
milliseconds (3 digits). Note that this patch also fixes three other bugs:

* CLJS-564
* The clojure reader reads "#inst \"2003-12-10T00:00:00.4\"" as #inst "2003-12-10T00:00:00.400" but the clojurescript reader (prior to
this patch) reads the same string as #inst "2003-12-10T00:00:00.004"
* #inst tests generated bad test data