Prefer std::variant in dataNode_/schemaNode_

Right now, I already got rid of using spirit alternative parser to parse
different types of nodes (leaf, container...), so no reason to use
boost::variant over std::variant.

Change-Id: I1f713427117d7135b8309b7129f3b025465a9f7d
diff --git a/src/parser.cpp b/src/parser.cpp
index 2942d56..195789b 100644
--- a/src/parser.cpp
+++ b/src/parser.cpp
@@ -88,7 +88,7 @@
         m_curDir = name;
     } else {
         for (const auto& it : name.m_nodes) {
-            if (it.m_suffix.type() == typeid(nodeup_))
+            if (std::holds_alternative<nodeup_>(it.m_suffix))
                 m_curDir.m_nodes.pop_back();
             else
                 m_curDir.m_nodes.push_back(it);