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/main.cpp b/src/main.cpp
index 3019d36..354114f 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -6,14 +6,13 @@
  *
 */
 #include <docopt.h>
-#include <experimental/filesystem>
 #include <iostream>
+#include <sstream>
 #include <optional>
 #include <replxx.hxx>
 #include "NETCONF_CLI_VERSION.h"
 #include "interpreter.hpp"
 #include "sysrepo_access.hpp"
-#include "yang_schema.hpp"
 
 const auto HISTORY_FILE_NAME = "netconf-cli_history";