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_path.hpp b/src/ast_path.hpp
index dcd13ff..a893e19 100644
--- a/src/ast_path.hpp
+++ b/src/ast_path.hpp
@@ -7,14 +7,10 @@
 */
 #pragma once
 
-#include <boost/spirit/home/x3.hpp>
-#include <boost/spirit/home/x3/support/ast/position_tagged.hpp>
-#include <boost/spirit/home/x3/support/utility/error_reporting.hpp>
-
 #include <boost/fusion/adapted/struct/adapt_struct.hpp>
 #include <boost/fusion/include/adapt_struct.hpp>
 #include <boost/fusion/include/std_pair.hpp>
-#include <boost/variant.hpp>
+#include <boost/variant/variant.hpp>
 #include <map>
 #include <vector>