Start of new ANTLR4 based parser.

Very simple tests pass.

pglablib/sqlast/SelectItem.cpp

@@ -0,0 +1,13 @@
+#include "SelectItem.h"
+#include "Expression.h"
+
+using namespace sqlast;
+
+SelectItem::SelectItem(std::unique_ptr<sqlast::Expression> expr)
+    : expression(std::move(expr))
+{}
+
+void SelectItem::SetAlias(const std::string &alias)
+{
+    this->alias = alias;
+}