Make use of C++17 std::optional<> instead of boost::optional<>.

test/libyul/ObjectParser.cpp

@@ -27,11 +27,11 @@
 
 #include <libsolidity/interface/OptimiserSettings.h>
 
-#include <boost/optional.hpp>
 #include <boost/algorithm/string/replace.hpp>
 
-#include <string>
 #include <memory>
+#include <optional>
+#include <string>
 
 using namespace std;
 using namespace langutil;
@@ -63,7 +63,7 @@ std::pair<bool, ErrorList> parse(string const& _source)
 	return {false, {}};
 }
 
-boost::optional<Error> parseAndReturnFirstError(string const& _source, bool _allowWarnings = true)
+std::optional<Error> parseAndReturnFirstError(string const& _source, bool _allowWarnings = true)
 {
 	bool success;
 	ErrorList errors;