Stop type lookups on lexically bound symbols

This is a port of @ztellman's pending patch http://dev.clojure.org/jira/browse/CLJ-1529

This patch avoids unnecessary lookups on symbols that are known to be lexically bound, resulting in significant speedup in many cases. Details are on the Jira page.

In the event of @ztellman's branch getting merged in to Clojure-JVM, merging this PR should reproduce its behavior.