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/parse_productions.cpp

@@ -1,7 +1,9 @@
-#include "parse_productions.h"
-#include <assert.h>
 #include <stdio.h>
+
 #include "parse_tree.h"
+#include "parse_productions.h"
+#include "parse_constants.h"
+#include "common.h"
 
 using namespace parse_productions;