Start of new ANTLR4 based parser.

Very simple tests pass.

pglablib/sqlparser/Parser.cpp

@@ -0,0 +1,21 @@
+#include "Parser.h"
+#include "antlr4-runtime.h"
+
+
+Parser::Parser(const std::string &input_string)
+    : InputStream(std::make_unique<antlr4::ANTLRInputStream>(input_string))
+    , CaseFilter(InputStream.get(), true)
+    , Lexer(&CaseFilter)
+    , TokenStream(&Lexer)
+    , AParser(&TokenStream)
+{
+    AParser.removeErrorListeners();
+    AParser.addErrorListener(&Errors);
+}
+
+std::unique_ptr<sqlast::StatementList> Parser::Parse()
+{
+    auto context = AParser.main();
+    return std::move(context->program);
+}
+