Streamline log enablement (#64)

* Remove log_enabled statements around annotate calls, which are already guarded against annotations_enabled

* Use a trace_enabled!() macro that follows the same logic as trace!() to find if additional traces have been enabled or not

src/ion/mod.rs

@@ -97,7 +97,7 @@ impl<'a, F: Function> Env<'a, F> {
         self.fixup_multi_fixed_vregs();
         self.merge_vreg_bundles();
         self.queue_bundles();
-        if log::log_enabled!(log::Level::Trace) {
+        if trace_enabled!() {
             self.dump_state();
         }
         Ok(())