- Tests do not pass yet because .clj, not .cljc

- All tests now pass, both CLJ and CLJS.
- However, do check the (very few) things marked “FIXME” and address
them as you see fit. They involve `eval` (CLJ only), a reflection test
(CLJ only), a cljs.test multimethod which is only available from CLJ,
and platform-specific stack traces in CLJS. None of these things alter
the functionality of Slingshot.

- In order to do this, had to drop 1.4, 1.5, 1.6 support. We keep 1.7
and 1.8 support. .cljx is an option if we need to have 1.4-1.6 support
still.

This fixes something that appeared when I tried to use the CLJS version
of Slingshot in a project — namely, that slingshot.support referring to
its own macro apparently sometimes causes a problem (though it doesn’t
always). Thus, the offending macros have been moved to slingshot.util.