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/schema.hpp b/src/schema.hpp
index 145ab07..199cb96 100644
--- a/src/schema.hpp
+++ b/src/schema.hpp
@@ -9,7 +9,7 @@
 
 #pragma once
 
-#include <boost/variant.hpp>
+#include <boost/variant/variant.hpp>
 #include <set>
 #include <stdexcept>
 #include <unordered_map>