Reset autoloads in response to variable changes

Prior to this fix, autoloads like function and completion autoloads
would check their path variable (like fish_function_path) on every
autoload request. Switch to invalidating it in response to the variable
changing.

This improves time on a microbenchmark:

    for i in (seq 50000)
      setenv test_env val$i
    end

from ~11 seconds to ~6.5 seconds.

src/complete.cpp

@@ -1558,6 +1558,8 @@ wcstring complete_print() {
     return out;
 }
 
+void complete_invalidate_path() { completion_autoloader.invalidate(); }
+
 /// Completion "wrapper" support. The map goes from wrapping-command to wrapped-command-list.
 static fish_mutex_t wrapper_lock;
 typedef std::unordered_map<wcstring, wcstring_list_t> wrapper_map_t;