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_context.cpp b/src/parser_context.cpp
index 26e6fa3..6b15cb9 100644
--- a/src/parser_context.cpp
+++ b/src/parser_context.cpp
@@ -43,7 +43,7 @@
 void ParserContext::pushPathFragment(const dataNode_& node)
 {
     auto pushNode = [this] (auto& where, const auto& what) {
-        if (what.m_suffix.type() == typeid(nodeup_)) {
+        if (std::holds_alternative<nodeup_>(what.m_suffix)) {
             where.m_nodes.pop_back();
             if (where.m_nodes.empty()) {
                 m_topLevelModulePresent = false;