add better support for IWYU and fix things

Remove the "make iwyu" build target. Move the functionality into the
recently introduced lint.fish script. Fix a lot, but not all, of the
include-what-you-use errors. Specifically, it fixes all of the IWYU errors
on my OS X server but only removes some of them on my Ubuntu 14.04 server.

Fixes #2957

src/function.cpp

@@ -6,9 +6,7 @@
   is taken care of by the parser and to some degree the builtin
   handling library.
 */
-
-#include "config.h" // IWYU pragma: keep
-
+// IWYU pragma: no_include <type_traits>
 #include <wchar.h>
 #include <pthread.h>
 #include <map>
@@ -17,10 +15,10 @@
 #include <stddef.h>
 #include <string>
 #include <utility>
+#include <memory>
 
-#include "wutil.h"
+#include "wutil.h"  // IWYU pragma: keep
 #include "fallback.h" // IWYU pragma: keep
-
 #include "autoload.h"
 #include "function.h"
 #include "common.h"