ref qualify parser methods to avoid use of dangling objects (#703)

To avoid using data belonging to a temporary, the parse functions are ref qualified to get a compile error if used on an rvalue. See https://github.com/simdjson/simdjson/issues/696

Compilation tests are also added, to make sure bad usage fails to compile.

Reviewed by jkeiser.

tests/compilation_failure_tests/dangling_parser_parse_stdstring.cpp

@@ -0,0 +1,16 @@
+
+// this tests https://github.com/simdjson/simdjson/issues/696
+
+#include <iostream>
+#include "simdjson.h"
+
+int main() {
+    std::string buf;
+#if COMPILATION_TEST_USE_FAILING_CODE
+    simdjson::dom::element tree = simdjson::dom::parser().parse(buf);
+#else
+    simdjson::dom::parser parser;
+    simdjson::dom::element tree = parser.parse(buf);
+#endif
+    std::cout << tree["type"] << std::endl;
+}