Clean includes and usings

These changes are based on suggestions made by the include-what-you-use
tool. Some suggestions were not implemented because they did not
decrease build time (forward-declaring some classes), or because they
would make the include section too complicated (Spirit headers and MPL
headers). On my computer, this change decreases the build time by about
27%:

before
real	1m45.807s
user	5m40.648s
sys	0m18.274s

after
real	1m18.013s
user	3m57.866s
sys	0m12.946s

Change-Id: I11e4f30f4e7518506a213537daae9c41d52b4ac4
diff --git a/src/ast_handlers.hpp b/src/ast_handlers.hpp
index 69af5ac..fd7f52e 100644
--- a/src/ast_handlers.hpp
+++ b/src/ast_handlers.hpp
@@ -9,8 +9,11 @@
 #pragma once
 
 #include <boost/mpl/for_each.hpp>
+#include <boost/spirit/home/x3.hpp>
+#include <boost/spirit/home/x3/support/utility/error_reporting.hpp>
+
+
 #include "ast_commands.hpp"
-#include "parser_context.hpp"
 #include "schema.hpp"
 #include "utils.hpp"
 namespace x3 = boost::spirit::x3;