Pass IdString by value instead of by const reference.

When IdString refcounting was expensive, it made sense to pass it by const reference
instead of by value, to avoid refcount churn. Now that IdString is not refcounted,
it's slightly more efficient to pass it by value.

kernel/modtools.h

@@ -161,7 +161,7 @@ struct ModIndex : public RTLIL::Monitor
 #endif
 	}
 
-	void notify_connect(RTLIL::Cell *cell, const RTLIL::IdString &port, const RTLIL::SigSpec &old_sig, const RTLIL::SigSpec &sig) override
+	void notify_connect(RTLIL::Cell *cell, RTLIL::IdString port, const RTLIL::SigSpec &old_sig, const RTLIL::SigSpec &sig) override
 	{
 		log_assert(module == cell->module);